Gasoline and water do not mix because they have different density levels, or mass per unit volume. Water has a density of 1 gram per milliliter, while gasoline has a density of 0.8 gram per milliliter.
Fill an object with gasoline and water and the water sinks to the bottom due to its higher density, and the gasoline rises to the top. Density also explains why salty bodies of water, such as the Great Salt Lake, are easy for people to float in. The density of saltwater is higher than regular water, making it easier for people to float to the top.