The short answer is that air molecules scatter the blue light from the Sun much, much more than other colors and in all directions. This makes our sky blue. Below is a more detailed explanation.

Sunlight is a mixture of all the colors of the rainbow. The colors in sunlight have almost the same intensity. Our eyes perceive this mixture as white light. CAUTION: DO NOT LOOK DIRECTLY AT THE SUN!!! Serious eye damage can occur!

Light is a wave, and red light has a longer wavelength (distance between peaks of the wave) than blue light. The molecules of air are much, much smaller than the wavelengths of visible light. Therefore, the air scatters sunlight in all directions. The blue light in sunlight is scattered much, much more than the other colors, making the sky appear blue. This explanation is called Rayleigh scattering, named for the 19th Century British physicist who discovered it.

Farts are a natural byproduct of eating a nutritious diet to supply our bodies with energy and nutrients needed for us to survive and thrive.  Farts and poop are the waste products of our digestive process, which extracts the needed stuff from our food.   

Why do farts (flatulence) smell so bad? The answer starts with the food we eat.  For example, eggs (very healthy food) contain sulphur (S), which combines with oxygen to give us energy. The waste product of digesting eggs is sulphur dioxide gas - sometimes called rotten egg gas because it smells like rotten eggs…ugh!  A lot of the food we eat (raisins prunes, meat products, vegetables, soft drinks, wine and beer, etc.) contains sulphur compounds.  A waste product of digesting that food is sulphur dioxide, which our bodies expel as smelly farts!

Dogs use their tails to communicate. For example, when your dog sees you, they wag their tail to communicate to you that they are excited to see you. It's a way of showing their affection for you. 

On the other hand, when a dog does something wrong and you scold them for it, the dog might stick its tail between its hind legs and slowly wag it. This communicates to you that your dog is feeling sorry or ashamed. 

However, just as our voices can communicate anger or fear in addition to excitement and affection, sometimes a dog wagging its tale is communicating that they are angry or afraid. You can probably tell the difference with your dog, but it's more difficult to tell with a dog you don't know. Never approach or touch an unknown dog unless their owner is present and gives you permission!

In science, it's important to ask the right question to get the best answer. We can better understand the answer to this question by changing the word "heat" to "light." So, let's look at this question: Since white reflects light and black absorbs light, why are the lenses in sunglasses usually black?

The answer is that we make sunglasses partly black to absorb some fo the sunlight coming through them. That way, the light that makes it through the dark glasses is not too bright for our eyes. 

Note that sometimes sunglasses look shiny white rather than dark. These glasses are made of very thin layers of metals (like aluminum) that reflect some white light but let some through. The light that comes through the lens is now safe for our eyes, because it's not too bright. 

Explore More! Words used in science have very specific meanings. For example, light is electromagnetic radiation. That radiation can be reflected (off a mirror), transmitted (through a clear piece of glass), or absorbed (into a black surface). The light absorbed by a black surface heats (raises the temperature) the surface. Light reflected or transmitted does not heat the surface it's interacting with. 

Crayons are made by mixing melted paraffin wax with a carefully measured amount of paint pigments to produce many colors. The mixtures of pigments and paraffin wax are then poured into special molds in the desired shape and allowed to cool and become solid.

A sudden shift or slip on a fault in the Earth’s crust causes earthquakes. The Earth’s crust is massive plates of rocky material. These plates are always slowly moving and become stuck. The pressure between the plates builds up until they shift of slip, releasing lots of energy causing the ground to shake violently.

Volcanoes occur when hot molten rock and metals (called magma when under the Earth’s surface) erupt through holes or cracks in the Earth’s surface, resulting in the flow of lava (molten rock and metals above the Earth’s surface).

The science of volcanoes starts with understanding the layered structure of the Earth. Starting at the center, the Earth is composed of four distinct layers. The study of these layers produces the science behind volcanoes. Starting from the center, the layers are the inner core (superheated solid iron and nickel), the outer core (liquid iron and nickel), the mantle (a mixture of solid and liquid metals and rocks), and the cool surface called the crust, where we live.

A process called convection heats the mantle and results in a mixture of liquid and solid metals and rock called magma. The hot, high-pressure magma rises toward the surface. In the last step of this process, the magma rises and erupts at the surface through cracks and fissures in the crust. We call these eruptions volcanoes. Lava is the flowing liquid rock and metals of the eruption.

Earthquakes or volcanic eruptions under the ocean release large amounts of energy, which in turn produce huge waves that can cross the oceans and flood coastal areas. The amount of energy released can be more than the largest atomic bomb. Ocean shores nearest the earthquake or erupting volcano experience much larger waves and flooding than those far away.

The color of fire is due to two things:  1. the temperature of the fire and 2. the material being burned.

How does temperature impact the color of fire? This question is the easiest to answer.  The hotter the fire, the bluer the color.  Blue-hot is much hotter than red-hot!  The physics behind this is called black body radiation.  

Light is electromagnetic radiation, and all objects radiate light depending on their temperature.   This is how “contactless” thermometers work.  Our body temperature is about 98-degrees Fahrenheit, which we radiate in infrared - well outside the range of human vision, but detectable by a thermometer.   If you build a wood fire, the burning wood will first radiate in the red range of our vision.   As the fire burns hotter (perhaps because we fan it), the hot coal left by the burning logs radiates blue light.

So how does the material being burned impact the color? There are particular materials that burn differently than wood (think fireworks). The gases from these burning materials emit light at many discreet colors or wavelengths.  The specific colors produced depend on quantum physics. 

Full question: "I understand that to combat global warming there is a push for green energy. One green energy production source involves the production of green hydrogen using electrolysis to split water into its constituent elements. Since much of the world is also suffering from a water crisis and the water cycle is a closed system (I think), how would the mass production of green hydrogen from water affect the water cycle and hence the availability of water??

The short answer is that using hydrogen fuel produces water as a byproduct. The net result is that no water disappears from the water cycle!

To dive a little deeper, given the answer above, one must question "green hydrogen." One can never get more energy out of hydrogen fuel than that required to separate hydrogen from water. Clean energy such as wind and solar provide electricity for electrolysis, which in turn produces "green" hydrogen - a transportable green energy. In other words, by storing solar and wind energy, green hydrogen is energy in a transportable form.

Expert scientists and trained technicians do genetic tests using specialized instruments. For example, a thermocycler can split and amplify DNA, allowing scientists to create multiple copies of the same strand. A gel electrophoresis machine takes film of the scientists’ findings to allow closer examination. You must supply a blood or spit sample for testing and results are usually ready in a few weeks.

Starfish can live up to 35 years. There are over 2000 species of starfish, many of which live in tropical areas like Florida coastal waters. They cannot live in fresh water.  One interesting fact about starfish is that they can regrow or regenerate a lost limb. 

Yes! There are all sorts of small rockets. For example, some scientists use small rockets to send probes into large storms, where they gather critical data about the storm. You have also probably seen very small rockets at Fourth of July celebrations. In fact, most firework displays consist of a small rocket used to shoot a payload high above your head. The payloads then explode in beautiful colors! You can read more about what gives fireworks their colors in my answer to the question “Why is fire different colors?”

Explore More! Did you know you can launch your very own small rocket? Small rockets, sometimes made to be models of larger rockets like the Saturn V used by NASA, are available at hobby stores and online. It’s important to learn all the proper safety precautions before experimenting with such rockets. Some model rockets carry payloads (like NASA rockets!) that can measure the height of the rocket and the temperature of the air. Make sure that an adult obtains such rockets for you and supervises their use!

Potential energy converts to kinetic energy according to Newton's laws of motion. These laws determine how quickly potential energy converts into kinetic energy. Newton had to invent a whole new area of mathematics called calculus to accurately describe this process - including the length of time needed for the conversion to happen! 

Let's consider an example to help demonstrate this. Suppose you pick up a rock and hold it at eye level - say, about 5 feet above the ground. This gives the rock potential energy. Your reaction force holds the rock steady, balancing the force of gravity (Newton's third law). 

Now, remove your hand. The force of gravity accelerates the rock towards the Earth's surface, changing the potential energy to kinetic energy, also known as the energy of motion. It takes a little over 1/2 of a second for the rock to reach maximum kinetic energy and maximum velocity of about 18 feet per second. So the kinetic energy of the rock just before it hits the ground equals the potential energy it had before you released it.

Note that the time it takes the rock to hit the ground and the maximum velocity of the rock do not depend on how much the rock weighs! The great Greek thinker and philosopher Aristotle thought that a heavier rock would fall faster than a lighter one. But he was wrong! Galileo did an experiment that proved this, and Newton discovered the laws of nature that explain why.

This is a great but complicated question! There are a lot of YouTube videos on the web that demonstrate how to make different types of paper airplanes. This one shows how to make the world-record-holding design: 

There are four forces acting on a paper airplane: thrust (how hard you throw it), drag (the force of air resistance slowing the plane down), gravity (the force pulling the plane to the ground), and lift (the force of air flowing over the plane's wing. These forces allow the plane to fly level or climb through the air. We can't exactly influence gravity, and there's not much we can do about thrust during the design phase. So, let's focus on lift and drag.

In general, the more pointed the plane, the lower the drag. Additionally, the larger the wing area, the larger the lift. Just like all airplanes, the best design depends on what you want it to do! To go straight and fast, make your plane very pointed with a narrow wing. If you optimize this design, your plane will fly fast and straight, but it won't fly for very long or go very far. For a plane that goes far, your design will have to have a larger wing - perhaps with a few twists to help make the most of the thrust (your throwing power). In both strategies, the wings swept back a bit to reduce drag.

Here is a design I learned when I was a child. Use a standard piece of computer paper, 8.5x11 inches.

1. Fold the paper back about 2.5 inches from the top. This fold will just about meet the midpoint of the 11-inch side of the paper.
2. Fold the paper along the middle of the long direction. Always try to keep your folds straight and crisp!
3. Now fold the thick part into two right triangles, with one side along the centerline and one side parallel to the bottom of the paper (the part without any folds in it).
4. Fold the pointy part about 1.5 inches towards the tail. Make sure the folds are crisp by re-doing the fold along the center.
5. Now make a fold about halfway along the (now blunt) point of the plane. Do these on each side of the center. The resulting V shape from the wings should be tilted, so that the V is slightly longer in the back of the plane.
6. At the very tips of the back of the wings, make a small triangular fold upward (only about a half inch).

I do not guarantee it will be the best plane, but I leave it to you to experiment with the basic steps above to improve it!

Scientists and engineers design and build airplanes to fly. Key parts of airplanes are wings (airfoils) and engines. The two types of engines are propeller and jets. Both push the plan forward by pushing air (for propellers) and hot gasses (for jets) toward the back of the plane. This pushes the plane forward by the equal and opposite forces explained by Newton's Third Law of Motion: for every action (force) in nature, there is an equal and opposite reaction. In other words, forces always act in equal but opposite pairs. 

The specially designed wings cause the pressure on top of the wings to be less than below the wing. The lift generated by this pressure difference allows the plane to maintain or gain altitude. 

Suppose the engines quit running. Does the plane fall out of the sky? No! The pilots can adjust the wing control structures to allow the plane to glide down to the ground, rather than plummeting straight down. 

Note: It's very rare for this to happen! Before every flight, pilots and ground crews go through an extensive checklist to make sure everything is working correctly. Most commercial airplanes (and some private planes as well) have more than one engine. If an engine quits running, then the pilots can safely direct the plane to an airport for safe landings with the remaining engine or engines. If all engines fail (very rare indeed!), then the pilots adjust wing control structures to glide down to a safe emergency landing.

Let's start with constellations. 

The important thing to know about constellations is that they have no physical or scientific meaning.

Constellations are areas in the sky (the celestial sphere) that were grouped and named by ancient people. Stars within a constellation usually have various levels of brightness and are different colors. In general, they are not near to each other and were arbitrarily assigned a name. The first people to do this were Babylonian astronomers around 2000 BCE (about 4,000 years ago). 

Many of the world’s cultures have made such groupings, often calling them by different names and sometimes including additional stars (or fewer stars) in their grouping.  For example, Native Americans call the Little and Big Dippers the Small and Large Bear.  The 48 “official constellations” in Western culture were grouped and named by the Greek astronomer Ptolemy in ACE 200 (about 1,800 years ago.) The names he assigned were from ancient Greek mythology.

So, how and why are stars classified? The answer is quite complicated, but here's the main idea. 

Star classifications help guide our understanding of the physics governing our Sun. More generally, they also inform our understanding of how stars form, live, and die.  This understanding is critical to further developing our understanding of our universe.   

The primary way stars are classified is by their surface temperature and luminosity, which is defined as their total radiative power or intrinsic brightness.   Temperature is the easiest thing to measure because it’s indicated by the peak color or wavelength of stellar emission.  One can get a good idea of a star’s temperature with your unaided eye!  

For example, one of the most prominent constellations in our sky is “the Hunter,” Orion. Orion is outlined by a large rectangular area of our sky, and on diagonal corners are Betelgeuse and Rachel. Betelgeuse (pronounce “beetle juice”) appears red and is relatively cool in temperature, while Rachel appears blue and is relatively hot. Using an instrument called a spectrometer, one can measure a star’s temperature very accurately.

To accurately calculate a star’s luminosity, one must know a star’s distance from us. Distance measurements are hard, but direct measurement of a star’s parallax (its apparent position relative to background stars at opposite positions in Earth’s orbit) is straightforward.  This method can be used to measure distances out to about 1,000 lightyears from Earth. More complex methods are needed for more distant stars.   Once we know a star’s distance, we measure its apparent brightness (how much light do we detect with our telescopes) and then calculate the star’s luminosity.

Another classification of stars is by how many heavy elements they contain. This is done by completing a very detailed analysis of the light (called the spectra) from the star.  In this manner, astronomers can determine the star’s relative age – i.e. if a star was formed near the universe’s beginning or from recycled material from older stars. The Sun, for example, is made up of about 3% heavy elements (star stuff from the life and death of previous stars.)

The short answer is very carefully! This is an important and complicated problem. For #2, they sit on a toilet seat like the ones we use on Earth. Then they use a device similar to a vacuum cleaner to suck up the waste, vacuum dry it, and send it down with other waste when they return to Earth. For #1, they pee into a tube connected to the vacuum-like-device that vents the liquid into space.

It is critical to get this process right, so astronauts have to train themselves to use these systems before they go into space!

The simple one-word answer is gravity! The gravity associated with the Earth’s mass attracts other matter.

Meteors are mostly sort of space junk (mixtures of rock and ice) left over from “dead comets” (comets broken apart by the Sun’s gravity) or left over material from the formation of our solar system.

Meteors from “dead” comets mostly follow the same orbit around the Sun as the comet did before it broke apart. Solar winds push some of the particles along the orbit of the former comet. When our Earth encounters this debris, we have a shower of meteors, sometimes hundreds per hour.

The short answer is yes! But let's test it for ourselves:

Test 1: The Refrigerator Magnet

Go to your refrigerator and pull off a magnet. Feel the force? That force is a measure of the strength of the magnetic field that holds the magnet firmly on the door. 

Test 2: Gravity

Pick up something from the floor and then let go of the object again, and watch what happens. It falls to the floor! The force you feel in lifting the object and the force accelerating the object when you let it go are both due to the Earth's gravitational field. 

But what about the "force fields" used in science fiction stories like Star Trek? The short answer is that no, force fields do not exist in the way they're depicted on these shows. However, we use high magnetic field to "contain" very hot, dense plasmas in experiments to do controlled fusion - which scientists are investigating as a solution to the clean energy problem. So in some ways, that concept is based in science. "The Force" in Star Wars, however, has no basis in scientific concepts.

All cars need a source of energy. Electric cars store energy from the electric power grid in large batteries. Like gas-powered cars, they have a pedal on the driver side floor. Pressing the pedal sends electric current to the car’s electric motor. The amount of current to the motor controls the speed of the motor and the force, called torque that turns the car’s wheels.

Science was probably invented by a young child driven by curiosity about the natural world. Note that science is a human effort to understand the natural world through observation and experimention. I would bet that you've done this yourself!

It's likely that the earliest science-related topics were explored in Egypt and Mesopotamia about 5,000 years ago. They contributed to understanding math, astronomy, and medicine. 

About 2,500 years ago in the 4th century BCE, Aristotle of Greece pioneered the application of logic, observation, and inquiry to help understand nature. However, the word "scientist" wasn't used back then. People who explored and attempted to explain the natural world were called "natural photospheres" instead.

Galileo Galilei, born on Feb 14, 1564 in Italy, was arguably the first scientist as we would define it: someone who studies nature through the application of logic, observation, and experimentation. For example, Aristotle reasoned that gravity accelerated heavy objects faster than lighter ones, but he never tested this hypothesis with measurement. Galileo proved by reason and experimentation that Aristotle was wrong; acceleration due to gravity is independent of the mass of the object. 

The first person to use the word "scientist" was William Whewell. He coined the term at Cambridge University in 1834, using it to refer to a person who explores and tries to understand the natural world.

Science is the knowledge and study of the natural world based on evidence gathered from experiments and observations. People that study and create art are called “artist.” Therefore, scientists are people that study and do science.

Human children are born curious, and as soon as they can, they start observing and experimenting to try to understand how nature works. Therefore, you are born a scientist!

But how long does it take to become a professional scientist?

You can get an associate’s degree (AA) in two years to become a science technician. The technicians in science work with more senior scientists to take data, setup equipment, and troubleshoot equipment in experiments or applications.

Those wanting to have a career doing scientific research must first attain a baccalaureate degree (Bachelor of Science or BS degree). This takes about four years of hard (but fun!) work and lots of math. While one can get a job as a scientist with a baccalaureate degree, it takes a PhD degree in the subject matter to be able to do pioneering research and lead research teams. This takes another 4 to 6 years of work and study. However, the work is rewarding, and the career opportunities are limitless!

The reality is that scientists never stop learning. One must keep up with advances and every new thing you learn in doing research presents new and exciting challenges. To be a scientist means a lifetime of learning.

It's interesting and fun! I have spent most of my life trying to better understand our world - and the better I understand the scientific basis of our world, the more I can enjoy living in it. 

As a career, being a scientist has been unlike anything I could have imagined. My employers and granting agencies paid for the tools (read: toys!) that I needed. I got to work with people from all over the world and travel to fun and interesting places. I made many new friends from many cultures. Colleagues treated me as a family member, and I learned firsthand how they lived, tasted the food and drink they enjoyed, and appreciated the values we shared. 

The ultimate joy is to learn something new! This is sometimes called the "ah-ha moment," a moment in which you know something that nobody else does. This is an experience like no other. It leads to a new beginning, the next steps found in the new questions that occur to you, again and again - until suddenly, you look back and realize you've completed an incredible journey. 

Then you get to teach others what you have learned, a process in itself that produces new learning experiences. The cycle never ends!

Gravity is one of the four basic forces of nature. The other three are the electromagnetic, weak, and strong fources. These four fundamental forces describe every interaction in nature. It is thought that these forces were combined during the very early part of the universe, when the universe was extremely hot and dense. As the universe expanded and cooled, the forces separated. 

Gravity is the weakest of the four forces. For example, the strong force is 1 x 10⁷⁰ times stronger than gravity. That's 10 with another 69 zeroes added at the end! However, the range of the strong force is limited to distances inside the nucleus of an atom. The range of gravity is infinite, and it interacts with anything that has mass. 

Scientists don't try to answer questions like "where does gravity come from?" Rather, scientists try to develop an understanding of how the force works and how it defines our physical world. Gravity is a force that acts on all matter; its strength is proportional to the masses that interact (such as the mass of a human and the mass of Earth). The strength of gravity is inversely proportional to the square of the distance between the two masses. Essentially, as the distance between two objects increases, the force of gravity between them decreases. Isaac Newton discovered this description of gravity in 1686. 

Newton's law of gravity is expressed in equation form as F = mMG/d², where F is the strength of the force, m and M are the masses of the two objects attracted by gravity, and d is the distance from each other. G is a universal constant of nature.

Einstein's General Relativity (1915) describes gravity as the curvature of space-time and has been verified by many experiments and observations. The mathematics used to describe General Relativity is very complex, requiring years of study to understand. 

Newton’s three laws of motion and law of gravity, published in 1687, revolutionized science and to this day enable the abundance that we enjoy in the developed world. Let us first look at the three laws of motion:

1^st Law: Objects in motion will remain in motion (at the same speed and in the same direction) unless acted on by a force. An alternative statement is an object’s momentum (its mass multiplied times its velocity) is conserved (does not change) unless acted on by a force.

2^nd Law: The acceleration (change of speed and/or direction) of an object is proportional to the net force applied to the object and inversely proportional to its mass. Equations are a sort of shorthand notation used in science, so this can be written as a=F/m. Note that a and F are in bold type, indicating that they are vectors. This means they have magnitude and a direction.

3^rd Law: For every action there is an equal and opposite reaction.

Newton’s Law of Universal Gravity:

Every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the distance between them. Let’s use our scientific shorthand (equations) to express this physical law. In equation form, the Law of Gravity is: F=mMG/d² where F is the force of attraction of two masses, m and M are two masses, d is the distance between the two particles, and G is the universal gradational constant (a fundamental constant of nature).

Newton’s laws revolutionized our understanding of the motion of objects. These laws explained the motion of the Earth, Moon, planets and their moons, comets, and all mechanical motion. Newton’s Laws are the backbone of the industrial revolution and our space program, and they enable the modern life we enjoy.