We learned that the seas have a lower freezing point than fresh water as well.
We gathered up a glass, poured in one cup of water, half a crayon (do not use the cheap waxy ones use crayola), 2 tablespoons of salt, and a spoon.
I told him to first place the crayon half in the clear fresh water.
It sank very quickly!
Next I had him stir in the salt and try it again.
The crayon did indeed float this time!
We talked about the fact that the salt water has more molecules than the cup of fresh water.
Remy thought this was awesome, too.