Question

A girl throws a rock into a pond. A leaf floating on the pond bobs up and down but does not change location on the pond's surface. Why not?

Answer 1

This is due to the difference in density. The rock is denser than the leaf. And also, the rock is denser than the liquid in the pond. If the material is denser than the other material, it will sink. The same holds true for the rock, it sinks. But when the material is less dense than the other material, it floats. And it holds true for the leaf, it floats.