Buoyancy - Boat And Rock
To illustrate buoyancy in a perhaps counterintuitive manner.
Fish tank with plastic "boat" and lead "rock."
Ask your students to vote on the outcome before doing the experiment.
You and a large rock float in a boat on a closed pond. Q: If you throw the rock out of the boat and it sinks to the bottom of the pond, does the water level in the pond (a) go up, (b) go down, or (c) remain the same? A: The water level in the pond will go down when the rock sinks in the pond.
C. Frank Griffin and Peter N. Henriksen, Physics challenges, TPT 18, 135-136 (1980).