Here's why:

* Density: Apples are less dense than peppers. Density is a measure of how much mass is packed into a given volume.

* Buoyancy: Objects float if their density is less than the density of the fluid they are placed in. Since water is denser than an apple, the apple floats. Water is denser than a pepper, so the pepper sinks.

Think of it this way: An apple has a lot of air pockets, making it less dense. A pepper is much more solid and therefore denser.