Here's why:

* Density: Gasoline is less dense than water. This means that a given volume of gasoline weighs less than the same volume of water.

* Buoyancy: Objects less dense than the fluid they are in will float. The buoyant force of the water will push the gasoline upwards, causing it to float on top.

Important Note: Gasoline is flammable and mixing it with water is a dangerous combination. Never pour gasoline into water.