Cartesian Diver

A small hollow glass figure about 45 mm tall that changes its buoyancy in water as the pressure around it changes. Placed in a closed, water-filled bottle, it sinks or rises depending on whether the bottle is squeezed or released, giving a clear, visual link between pressure, volume and density. In physics lessons it demonstrates fluid pressure and buoyancy and offers a hands-on introduction to Archimedes' principle and Boyle's law. Simple and engaging, it sparks curiosity and helps pupils grasp these key physical laws.