Here's why:

* Polarity: Water is a polar molecule, meaning it has a positive and negative end due to uneven sharing of electrons. Petroleum ether, on the other hand, is a non-polar solvent, composed of hydrocarbons with very little polarity.

* "Like dissolves like": Polar solvents tend to dissolve polar substances, and non-polar solvents tend to dissolve non-polar substances. Since water and petroleum ether have vastly different polarities, they don't mix.

* Density: Petroleum ether is less dense than water, meaning it will float on top of water.

Therefore, when you mix petroleum ether and water, they will form two distinct layers, with the petroleum ether layer on top.