All you need for this experiment is a bottle of water (at least 1 liter) and a ketchup packet.
Make sure the bottle is full to the top with water. Then add the ketchup packet and screw the lid on. When you apply pressure to the bottle the ketchup packet will rise and fall depending on the amount of pressure applied. The reason is that there is a small air bubble inside the ketchup packet and when you apply pressure to the bottle the air bubble compresses (since gases compress more easily than liquids). The compression of the bubble increases the density of the packet causing it to sink. When pressure is released the air bubble increases in size and the packet decreases in density causing it to rise again. Pretty cool!
ages 4 – 8