Cartesian Diver / Squiddy

The diver obeys pressure from outside the bottle. Ask students to predict what will happen. Squeeze bottle to make diver sink; release to make it rise. Key idea: Pressure compresses trapped air, increasing average density. Ask: How can squeezing the bottle move something inside? Safety and cleanup: Inspect bottle for leaks. https://en.wikipedia.org/wiki/Cartesian_diver