The Boat and the Plank-Brain teaser
Riddle
A boat is floating in the water, and a heavy plank of wood is on the boat. If the plank is thrown overboard into the water, what will happen to the water level?
The water level will drop. When the plank is on the boat, it displaces a volume of water equivalent to the weight of the plank. When it's in the water, it displaces only a volume equal to its own volume, which is less than the volume displaced by its weight.