What You Need:
- Plastic soda bottle
- Warm water
What You Do:
- Place a coin over the top of the bottle. Choose the coin that fits best over the bottle's opening and most tightly seals the bottle. Keep this coin handy for the experiment.
- Place the empty bottle in the fridge for about an hour, so it is really cold. (You don't need to put the bottle in the freezer.)
- Remove the bottle. Dip your hand in water and add a few drops of water to the rim of the bottle. Place the coin on the wet rim. The water will form a seal between the coin and the rim of the bottle.
- Hold the bottle in two hands and slowly lower it into warm water. Make sure you keep the bottle level, so the coin doesn't fall off. Your coin should start dancing. If it doesn't, check the seal to make sure there is water between the coin and the rim of the bottle.
The water warmed the air in the bottle. When air is warmed up, its entropy increases and the molecules (tiny particles) that make up the air move faster and away from each other, to become more disordered. This made the air expand, or swell up. When the air in the bottle swelled up enough, it pushed up on the coin and some of the air escaped. The coin dropped back down and sealed the mouth of the bottle again. Then the air began to swell up some more, and each time the coin lifted and dropped back down, more of the air escaped.