Empty 2 liter soda pop bottle with the cap
Hot tap water
1. Turn on the hot water tap and get it to be quite hot. Don't burn yourself but the hotter the better.
2. Fill the bottle about 1/4 full with the hot tap water.
3. Swirl and shake the bottle.
4. Pour out the water and put the cap on the bottle as fast as possible.
5. Take the bottle and stick it in the freezer.
6. Let it sit there for at least 30 minutes. The longer the better, however. Feel free to let it sit there overnight.
7. Take it out of the freezer and take a look at it. You should notice that the bottle will be slightly crushed.
When you swirl the hot water around the bottle, you are heating up the air inside the bottle. Hot air has air molecules that are moving fast. They are moving so fast that many of them leave the bottle. When you cap off the bottle you stop air flow from getting back into the bottle. As the air in the bottle cools, the air molecules slow down and push less hard on the sides of the bottle. the colder they get, the less hard they push. This allows the air pressure outside the bottle to push in and eventually squish the sides of thebottle.