In most instances, when you combine liquids, you end up with something that is the sum of its elements, rather than two separate substances. However, sometimes one liquid floats above the other, like when you combine vodka and water.

Specific gravity tells us which items are heavier when compared side by side; for example, wood floats because it has a lower specific gravity than water. Vodka has a specific gravity of 0.96, while water is 1.00 — so in theory, this means vodka should float on water.

You can see for yourself that vodka is lighter than water if you place a glass of water face-down atop a glass of vodka with a card in between, allowing only a little contact between the liquids. After about half an hour, the vodka will be in the top glass and the water in the bottom.