Watch for icebergs and see them disappear.
A few days ahead, cut the tops off some cardboard milk cartons. Let your kids fill them halfway with water, then freeze them solid. Remove the chunks of ice from the containers and toss them in the pool. Icebergs! The kids in our pool were amazed that something so heavy actually floats. They also liked watching it slowly, slowly, slowly melt. Gracie, 2, marveled, "It's disappearing!" Six-year-old Will had bigger plans: "Let's play Antarctica!"
Ask the kids... what's different about the water now that it's frozen. (Aside from being colder and hard, it's grown in volume and become opaque.)
What's going on? Ice floats because when it freezes it expands and becomes less dense than the water it displaces.