This is a variation on the
Cartesian diver, donated by Joe Snider. A plastic lemon
partially filled with sand floats in a sealed jar of
water. Pressing on the jar lid compresses the lemon,
reducing its volume and thus the buoyant force on it, and the
lemon sinks. When the lid is released, the buoyant force
again exceeds the weight of the lemon and it rises.
This illustrates the transmission
of pressure through liquids, Archimedes' principle, and
Boyle’s Law.
