What you Need
What to do
Put the lid on your bottle, and then put it in the freezer for half an hour or so.
Have a look at it has anything changed?
Open the bottle while it is still in the freezer so it doesn't warm up.
Cover the coin in a layer of tissue, and then make it wet.
Place the coin on the bottle with the tissue between the coin and the top of the bottle.
Leave the bottle for another half an hour until the tissue freezes
Take the bottle out of the freezer point it away from you and warm it up using your hands.
What may happen
You should find that the closed bottle shrinks as it cools down.
When you take the bottle out of the freezer and warm it up the coin should pop off. If it doesn't the tissue might not have made a good seal so you might want to try another one.
Why does it happen?
When you cool air or any other gas it shrinks, so the closed bottle is crushed by the air outside it.
If the bottle is open as the air shrinks extra air is pushed into the bottle.Then the wet coin seals the top of the bottle as it freezes.
This means that when you heat up the air again it pushes outwards harder and harder until the pressure on the bottom of the coin is large enough to break the ice around it. As the ice breaks the coin is pushed upwards and flies away from the bottle.
Why does air expand when you heat it up?
Air is made up of small particles which are bouncing off one another and the walls of the container they are in. These collisions push outwards and cause a gas to take up space. A hotter gas has more energy so the particles move faster and bash into the walls faster and more often so the gas pushes harder and takes up more space. This pressure pushes the coin off the bottle.
Try changing the temperature of the gas in the balloon (courtesy of daveansell.co.uk)