Coin poppers is a brilliant experiment to demonstrate air pressure!
What you need:
A small empty plastic drink bottle
A 2p coin
A small square of kitchen roll
1. Put the empty bottle in the freezer for about an hour to cool.
2. Cut a small square of tissue paper and put it on top of the 2p piece
3. Pour a little water over the tissue to stick it to the 2p
4. Without removing the bottle from the freezer, place the 2p on top of the bottle, tissue side down, to seal as a lid.
5. Leave for half an hour
6. Bring out and warm the top of the bottle with your hands!
7. Watch the magic!
The science bit
We assume we have put an empty bottle inside the freezer but of course it is actually full of air which is a mixture of gases, containing nitrogen, oxygen and carbon dioxide amogst others. As a gas cools it shrinks lowering the pressure, but becasue the lid is off, more air can enter the bottle from the freezer. Once you placed the coin lid on, you have sealed the top. As the air inside warms up again from your hands, it expands, and forces more pressure on the inside of the bottle and the lid,compared to outside the bottle. It makes enough pressure to break te seal and pop the lid.