Beakman's World: Sink or Float
Beakman demonstrates that marshmallows are less dense than the same amount of water and therefore, marshmallows float. Nails are heavier than the same amount of water so they sink. He then explains water displacement by getting into a tank of water. His DIY demo for this is making a aluminium boat that floats in a bowl of water but when you crush the aluminum foil into a square, it sinks because it couldnt' displace the water.