Make some salt water by adding a couple of teaspoons of salt to two cups of water. Fill an ice tray with this water. Fill another tray, but for this one use tap water only. Put them in the freezer and check them after a few hours. Float an egg in both salt and fresh water.
Salt water is more buoyant.
From that you might go on to discuss salt in the Dead Sea and how easy it is for people to float there. Let your child make a mark to show the water level. Use another dish with an equal amount of water and put this one in the shade. Watch what happens, and talk about what you see. This activity provides a memory jogger.