A boat with a man and some stones in it is floating in a pool of water. The man in the boat starts throwing the stones one by one in the water. What will happen to the level of water when he has thrown all stones in the water?

When the boat with the stones was floating in water, a volume of water was being displaced the mass of which was equal to the combined mass of the boat, the man and the stones. When the stones are thrown in water, the boat is displacing a volume of water; the mass of which is equal to that of the combined mass of the man and the boat. Clearly, this volume is less than what was being displaced before. Now, when the stones are thrown in water, they displace a volume of water equal to their own volume. Because of the fact that they sink in the water, the mass of this volume of water must be less than their masses. But when they were floating in the boat, a volume of water had to be displaced the mass of which was equal to that of the stones. Clearly, the volume of water displaced when the stones were in the boat was more than when it is empty. Hence, the water level comes down.