This week's experiment is a combination of two past experiments. We are going to combine the idea of the Dancing Raisins with the Cartesian Diver. You will need:

- a bottle of clear, carbonated soda. It needs to have a screw on cap.
- raisins

First, if the bottle of soda has a label, remove it so you can see inside. Carefully remove the cap. Drop in 5 or 6 raisins and quickly put the cap on tightly. Watch the raisins. They should sink to the bottom, with lots of tiny bubbles rising from them. After a few seconds, one or more of them will probably begin to rise. As soon as a raisin begins to rise, give the bottle a good, hard squeeze. As you do that, you should see the raisin begin to sink again. Release the pressure and the raisin begins to rise again.

Why does it do that? Lets take it one part at a time, starting with the bubbles. Where are the bubbles coming from? From the soda, right? The soda has carbon dioxide gas dissolved in it. This gas escapes, forming bubbles.

OK, so why do more bubbles form on the raisins? When you drop the raisin into the soda, its wrinkled skin traps lots of tiny air bubbles. These bubbles act as a starting point for more bubbles. As carbon dioxide gas moves from the soda to the bubbles, they get larger and larger. When they get large enough, the raisin begins to float.

Why does the raisin float? For that matter, why does anything float? If something weighs less than the same volume of water, it will float. For example, a cubic foot of Styrofoam weighs less than a cubic foot of water, so it floats. A cubic foot of steel weighs more than a cubic foot of water, so it sinks. The raisin is denser than the water, so it sinks. When the bubbles form, the combination of raisin and air weigh less than the same volume of water, so they begin to float.

OK, so far, so good. If you don't disturb things, the raisin will rise to the surface, where some of the bubbles will probably pop, letting the raisin sink again. But what happened when you squeezed the bottle? Why did the raisins sink? At first, you might think that you shook enough bubbles loose to let it sink again, but as soon as you stopped squeezing, it began to rise again.

So what would squeezing do? Squeezing the bottle makes it smaller, which means that the stuff inside has to get smaller. You can't make water smaller by squeezing it, so that means the air inside the bottle gets smaller. If you watch the top of the bottle, you can see the air space at the top getting smaller. The bubbles also get smaller. The smaller bubbles still weigh the same, since they contain the same amount of air, just squeezed into a smaller space. That makes them denser, letting the raisin sink. That is the idea behind the classic science experiment called the Cartesian Diver, which is the reason I called this experiment the Cartesian Raisins.

After a short time, you will notice that the raisins stop rising. Now what is wrong? Nothing. As the pressure builds up inside the bottle, the bubbles stop forming. All you need to do to get things going again is loosen the top. As soon as you hear the hiss of escaping air, you will see a burst of bubbles and some of the raisins will almost shoot to the surface. Tighten the cap and you will have another minute or two to play with the Cartesian Raisins.