Imploding can
This is one of the classic pressure demonstrations. You start by taking a normal empty drinks can and putting about 20ml of water (H2O) in (enough to cover the bottom). You then need to heat the can, as this causes the water to evaporate (going from a liquid to a gas). This means that the water expands with the molecules of water moving 1,700 times thither apart. This pushes the air that was in the can out.
The can is then turned upside down in cold water, the colder the better. This makes the water steam (water vapour) condense and take up less space. Because it is in water which forms a kind of seal, the air can’t get in to take up the space. This intern causes the air outside of the can to push on the can, until the it cannot take the force anymore and gives way.