To stack liquids without mixing, you would put the liquid with the higher density at the bottom and the liquid with the lower density on top. Density is a measure of how compact a substance is, or how much mass it has packed into a given volume. Under the influence of gravity, denser liquids tend to sink below less dense liquids. Therefore, by arranging liquids with higher density at the bottom and lower density on top, you can prevent them from mixing.
