Why might a fluid sit on top of another fluid?

A fluid can sit on top of another fluid due to differences in density and immiscibility. The less dense fluid, such as oil, will float on top of the denser fluid, like water, because buoyancy causes lighter substances to rise. Additionally, if the two fluids do not mix, such as oil and water, they will form distinct layers, with the lighter fluid on top. This phenomenon is commonly observed in various natural and industrial processes.