A man is sitting in a boat floating in the pond. If the man drinks some water from the pond, then what will happen to the level of water in the pond?
When the man sitting on a boat drinks water, the level of the water tend to decrease as a result of decrease in the volume of water in the pond. As the man gulp down the water, his weight will also increase by mass m. Due to increase in the weight of the man, the boat will sink a little and displace the the same volume of water drank by the man. Hence, the level of water in the pond raised is equal to the level fallen when the man drank the water.
Thus, the level of water will remain unchanged.