Cartesian Diver
The diver obeys pressure from outside the bottle.
Teaching & key ideas
What you do: Squeeze bottle to make diver sink; release to make it rise.
What students see: The diver obeys pressure from outside the bottle.
Why it works: Pressure compresses trapped air, increasing average density.
Question to ask: How can squeezing the bottle move something inside?
Lesson flow
Predict → demonstrate → observe → briefly explain → ask: How can squeezing the bottle move something inside?