Here's why:

* Density: Density is a measure of how much mass is contained in a given volume. Denser substances have more mass packed into the same space.

* Floating: Objects float if they are less dense than the fluid they are in.

* Oil and Water: Oil is *less* dense than water. This is why oil floats on water.

Think of it this way: If you put a rock (dense) in water, it sinks. But if you put a piece of wood (less dense) in water, it floats. The same principle applies to oil and water.