Here is a simple experiment that we adapted for our preschooler. When we had a blizzard last December, in addition to making snow cones with juice concentrate, we filled our graded measuring cup so that there was about 2 cups of unpacked snow. We talked about how snow is made of water, and that when the snow melts it will take up less space. This means that it will have less volume. We also talked about how some snow is more dense than others, and that this snow was especially light and fluffy. It was an introduction to the vocabulary and concepts and he seemed to think that it was interesting. I am going to do this experiment next time it snows and compare the results.