Most people are familiar with three states of matter: solid, liquid and gas. Actually, if you dig into the world of physics, there are several more, but for now we will only add plasma to the list, and we will look at the first three states before talking about plasma.

Solid

Things like rocks, wood and ice are solid. Solids stay the same size and shape, no matter what container we put them in.

Liquid

Things such as alcohol, oil, and water are liquids. They stay the same size, but they change their shape to fit their container.

Gas

The most common gas, the air, is actually a mixture of several gases. Gases change their shape to fit their container, just as liquids do. They also change their size to fill their container.

Plasma

The fourth state of matter is called plasma. Do not confuse this plasma with the plasma in blood. That is something completely different. Plasma as a state of matter is similar to a gas. It changes it's size and shape to fit a container. The difference is that in a plasma, each of the atoms has lost its electrons. These free electrons are moving around between the atoms. For this reason, plasmas are good conductors of electricity. Plasma also gives off light, which makes it easy to see.

This week's experiment comes from an email I got from a student. (Thanks Darius!) He wanted to know what causes your knuckles to pop. To try this, you will need:

- your hands

First, not everyone's knuckles will pop, and some pop more than others. Other joints in your body may also pop, some for the same reason as your knuckles, and some for other reasons.

There are different techniques for popping your knuckles, but they all work in basically the same way, by forcing the two bones of the joint to move farther apart. Probably the simplest way to do this is to interlace the fingers of your hands. Then turn your hands so that your palms are away from your body, and gently bend your fingers backwards. THIS SHOULD NOT HURT. DO NOT FORCE YOUR JOINT TO THE POINT WHERE IT IS PAINFUL. As the pressure builds, you should hear a pop from one or more of your knuckles.

If you search the internet for the answer to this question, you will find all sorts of answers, ranging from good explanations to wild guesses and pure fiction. The most comprehensive answer that I found was from "A bioengineering study of cavitation in the metacarpophalangeal joint" by
A. Unsworth, D. Dowson, AND V. Wright, from the Bioengineering Group for the study ofHuman Joints, the University of Leeds. If you want to read the article, you can find it here: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1005793/

If you don't want to wade through the article, the sound is caused by a process called cavitation. OK, so what is cavitation?

Lets think for a moment about the properties of liquids. Liquids take on the shape of their container, but they maintain their volume. You can't squeeze water into a smaller space, or stretch it to fill a larger space. If you try to stretch water into a larger space, at first, nothing seems to happen, but you are reducing the pressure of the liquid. When the pull gets strong enough, and the liquid pressure is low enough, some of the liquid changes to a gas, forming a bubble. Unlike the liquid, the gas in the bubble can be stretched into a larger space. That reduces the pull on the liquid, which raises its pressure. At that point, the gas in the bubble almost instantly changes back to a liquid, collapsing the bubble.

The same thing happens when you pop your knuckles. As you apply pressure to your knuckle joint, it forces the ends of the bones apart. Surrounding the joint is a liquid called synovial fluid. Moving the bones apart pulls on the fluid. If it was a gas, it would expand to fill the space as the bones separate. Since it is a fluid, it stays the same size, and its fluid pressure decreases. As you pull harder, the fluid's pressure gets lower and lower, until it reaches the point where the pressure is low enough to let some of the liquid change to a gas, forming a bubble. The bubble expands in response to the pull, which lowers the stress on the fluid. That lets the fluid's pressure go back to normal, which lets the gas change back into a liquid, and the bubble collapses. That collapse produces a loud sound.

Cavitation can be seen in other situations where liquids are subjected to very low pressures. Submarines have to be careful about cavitation from their propellers, as the sound can give away their position. An animal called the Pistol Shrimp snaps its claw fast enough to cause cavitation, producing a sound that is loud enough to stun or kill nearby fish.

Another question that frequently pops up with this subject is why you can usually only pop a joint once, and then you have to wait 20 to 30 minutes before it will pop again. When the low pressure causes the bubble of water vapor, it also causes some of the dissolved gases in the fluid to form bubbles. These bubbles, made up mostly of carbon dioxide, do not instantly collapse, remaining until the gases are reabsorbed back into the fluid. This usually takes 20 to 30 minutes. If you try cracking the joint again before those bubbles are reabsorbed, the bubbles of gas expand, preventing the pressure buildup that causes cavitation.

The other question that comes up frequently is whether knuckle cracking causes arthritis or other damage. Not many studies have been done, but those that have been done do not show any link between knuckle popping and arthritis.

Have a wonder-filled week.

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The Wonders of Ice

Explore why ice is one of the strangest solids we know of.

This week's experiment came from a conversation that I had with a friend about how things react to hot and cold. This is an easy way that you can observe what is going on as you change the temperature of some air. You will need:

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We are used to thinking of things as falling into the basic groups of solids, liquids, and gases. (In another experiment we will discuss a fourth state of matter, plasma.) In this experiment, we will examine a substance that sometimes acts like a solid and at other times acts like a liquid.

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This week's experiment came to me from my good friend Bob Cox. He told me about the trick and wanted to know why it worked. It took some thought and testing to come up with a theory of what is happening and then several e-mails to experts to confirm that I was on the right track. For this week's experiment, you will need:

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Can you lift an ice cube out of a glass of water with a string? Try it and see.

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Learn the science behind the Tricky Bottle!

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A cool way to see evidence of subatomic particles.

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Ice Crystals

Grow your own ice crystals.

Learn some fun tricks with dry ice as we explore the science of sublimation.

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Who would think that there could be so much science in a pot of boiling water?

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Half Full or Half Empty?

Is the glass half full or half empty? Neither!

When is a change of state not really a change of state?

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