Here's why:
* Density is a measure of how much mass is contained in a given volume.
* Water has a density of about 1 gram per cubic centimeter (g/cm³).
* Oil has a density that typically ranges from 0.8 to 0.9 g/cm³.
Because water has a higher density, it will sink below oil when the two are mixed. This is why oil floats on water.
