Cartesian Diver
The 'diver' sinks to the bottom of the bottle when one gently squeezes the sides of the bottle because pressure is applied and compresses the air bubble inside the 'diver' which will increase the density of the 'diver'. When the 'diver' is in the bottle and the bottle is not being squeezed, the air bubble is large enough such that it makes the 'diver' float.