Start the discussion of density and buoyancy by obtaining a clear glass or pitcher to use in your demonstration and filling it with water. Hold up two objects of similar size, such as a die from a board game and a piece of cork. Put them both in the water, and ask the kids why one sinks and the other floats. Point out that although they are the same size, one weighs more than the other, because it is denser. Explain the relative density of the plastic die, the water and the cork.
Hold up an egg and ask the class to predict whether it will float or sink. Place it in the water and let go to determine who's right. Retrieve the egg from the bottom of the glass, and ask your class, "What's the best way to make the egg float: reduce the density of the egg, or increase the density of the water?" Ask how water could be made more dense, and see how many suggestions are offered. Add salt one tablespoon at a time and stir it in until the water is dense enough to float the egg.
There are a number of ways to vary the experiment to keep the kids interested and thinking about the principles involved. Ask them what would happen if you began adding water to the glass. Would the egg sink again? How much water would it take? Ask them if they think sugar would work the same way. Perform the experiment again, this time adding sugar one spoonful at a time. Does it take more sugar than salt? What does that say about the relative density or salt and sugar? Would adding corn syrup to the water also work? Have them guess, then try it and see.
Buy eggs a month ahead of time and date the box. Buy fresh eggs at the time of the experiment. Set up two glasses beside each other. Fill them with water, and put a fresh egg in one and the older egg in the other. Does the older egg look different? Does it stand up on one end? Start adding salt to each glass one spoonful at a time. Which egg floats first? Explain that over time water evaporates from the egg through its shell, giving it less weight in the same volume and making it less dense.