Fill a large plastic container with water and place it on a table in the your home or preschool classroom.
Gather preschoolers around and give each one an object to use in the demonstration. Give some children pebbles and others plastic floating toys.
Ask each child to drop her object into the tub of water one at a time. Ask if anyone can explain why the pebbles sink and the toys float.
Explain that the pebbles are denser than the toys, which also makes them heavier. Tell preschoolers that stone is heavier than water so it sinks to the bottom. Meanwhile, the air inside floating toys is lighter than the water, which makes the toys buoyant and keeps them above the surface.
Teach children that their bodies are made up of a combination of air and dense solid material, so when they enter water they can sink and float. Explain that if they know how to swim, they can keep their heads safely above the surface, but if they don’t know how to swim they could sink below the surface like the pebbles and be in danger.