Get a small rock and cut a marshmallow to about the same size. Ask the student to lift both and ask which is heavier. A rock the same size as a marshmallow weighs more, so it is more dense than a marshmallow. Ask the pupil to predict what will happen when both are placed in water, then do so. The rock is more dense than water, so it sinks in water. The marshmallow is less dense than water, so it floats in water.
Pour an inch of vegetable oil into a clear glass. Have the student put a finger in the oil and taste it. Pour about 3 inches of water into the glass. Ask the child to place his finger in the top layer and identify whether it is oil or water The oil floats on top of the water because it is less dense than water.
Place 10 oz. water in a tall clear glass. Ask the student if he thinks an egg will float or sink in the water. Carefully lower the egg into the water. Remove the egg. Add 1/2 cup dry measure of sugar to the water and stir until dissolved. Ask the student if the egg will sink or float in the water. Carefully lower the egg into the sugar water. The egg floats because the sugar water is more dense than the egg.
Fill the sink at least 8 inches deep with water. Ask the student if he thinks a can of diet soda will float or sink in the water. Place the can of diet soda in the water. The diet soda has no sugar, and the soda is is about the same density as water. There is a little bit of air in the can. The air is less dense than the water. This is enough to make the can of diet soda float.
Ask the student to recall that the egg floated in sugar water. Tell the pupil that regular soda has a lot of sugar in it. Ask the student to predict if a can of regular soda will float or sink in the water. Place the can of regular soda in the sink. Regular soda sinks. Explain that this is the same reason the egg floated in the sugar water, because sugar water is more dense than the egg.