Here's why:

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

* Gravity: Gravity pulls denser substances to the bottom.

* Immiscibility: Liquids that don't mix are called immiscible. They resist dissolving into each other.

Example: Oil and water are immiscible and have different densities. Oil is less dense than water, so it floats on top of water, forming distinct layers.