The reason is that when fish are put into the water, the water surface will rise, so the more fish are put, the more the water surface will rise! If there are more fish in fish tank A than in fish tank B, the water level in fish tank A will rise much more than that in fish tank B, but the final result is that the water level in two fish tanks is the same, that is to say, the water level in fish tank A is lower than that in fish tank B, so fish tank B has more water.