Density

A heavy wooden block floats in water but a much lighter stone sinks. Why? Well, the stone is denser than the wood. The molecules that compose them are closer together in the stone than in the rock. Think of it like a storage container. One is packed with stuff when the other has a smaller amount of stuff shoved in. The one with more stuff is the exact same size as the one with less stuff, but weighs more. Why? Density. The stuff is much more densely packed into that unit. Density is mass per unit of volume. A stone weighs less than the wood, but is more dense, so it sinks. The wooden block is heavier but is less dense. http://www.physics.ucla.edu/k-6connection/Mass,w,d.htm