All of the packing and unpacking helped me decide which experiment to use this week. To prepare the equipment for the rough handling that airline luggage often gets, I use plastic bags filled with old towels to pad delicate parts and keep things from banging around inside the suitcases. The plastic bags often contain quite a bit of air, making it impossible to close the suitcase until the air is adjusted.

To see this happens, you will need:

- a jar or large glass with an opening large enough for your hand to fit into
- rubber bands
- a plastic bag large enough to fit over the jar. Be sure that it does not have any holes in it.

Lets start by putting the bag into the jar. Just stuff it in. It should fit easily. Next, remove the bag. Again, it is easy. You just pull on the bag and it comes right out.

For the next step, open the bag as if you were going to put things into it. Place the top of the bag over the opening of the jar. Use one or two rubber bands to secure the opening of the bag over the opening of the jar. Be sure that there is a good, tight seal all the way around, so that no air can get in or out. Once that is done, all you have to do is push the bag into the jar. Easy, right? Try it.

As long as there are no holes in the bag and there is a tight seal around the jar, you will not be able to push the bag inside. If the bag is big enough, you may get part of it inside, but you will reach a point where no more goes in. Pushing one part in will push another part back out. The air inside takes up space and with the bag sealed over the jar there is no place for it to go. As you push the bag inwards, you are squeezing the air into a smaller space. This increases the air pressure. When the air pressure inside the bag gets higher than the air pressure outside, it pushes the bag outwards until the amount of space on the inside of the bag is back where it started and the pressure is equal inside and out.

Now, lets reverse things. Remove the rubber bands and take the bag off of the jar. Place your hand inside the bag and push it into the jar, leaving the opening of the bag sticking out. Fold the bag outwards over the jar and seal it again with the rubber bands. Again, be sure that there is a good seal all the way around the jar, so that no air can get in or out.

When you have the rubber bands in place, try to take the bag out of the jar. Reach inside and pull the bag out. Again it sounds easy, but you will quickly find that it is easier said than done. As with our first effort, as long as there are no holes in the bag and a tight seal around the jar, you will not be able to accomplish your goal. Pulling out one part of the bag will pull another part back in.

Actually, that last sentence is a bit misleading. When you pull one part of the bag out, the other part is pushed back in, not pulled. It is pushed by the air pressure outside the jar. As you pull the bag outwards, you make the space inside the bag larger. Increasing the space lowers the air pressure inside the bag. Since the air pressure outside the bag is now higher, it pushed the bag inwards until the amount of space inside the bag is back where it started and the pressure equalizes again. At this point, I have a bit too much space inside me and a bowl of butter pecan ice cream has my name on it.