A small boat is floating in a swimming pool. The boat contains a very small (very very very small) but very heavy rock. If the rock is tossed out of the boat into the pool, what happens to the water level in the pool?
It is related to Physics, but also common sense. A floating object displaces water based on mass; a sinking object displaces water based on volume. So, at the beginning, the boat is floating, the water level rises by the mass of the heavy rock. If the rock is tossed overboard, the water level will drop by the mass of the heavy rock and rise by the volume of the heavy rock. Since the volume of rock is very small, the net result is the drop in water level.