The place where the lower end of a river meets the ocean is called an estuary (ES-choo-AIR-ee). What do you think happens when the rush of freshwater collides with the gush of the ocean's tides? Do the two kinds of water instantly mix? Does the freshwater float on top of the saltwater? Or does the saltwater float on top? Try this activity to discover the answer for yourself. Overview: In this activity, you'll compare three solutions with different levels of salinity, or salt. By using food coloring, you'll be able to observe which solution is the most dense, and which is the least dense. You'll also investigate how a liquid's density affects objects that are put into it.