In the first round, I handed each pair no students a different material to compare its mass versus volume. Then, in the second round, I had the entire class do samples of aluminum. Round three was water. And in the final round I did carbon dioxide as a demo. But only after they had to guess the density.
They great thing was at the end of eighty minutes, everybody won. We defined the relationship of mass to volume in a “for every” statement. For water we said for every 1.0 g we had 1.0 ml. Then we compared the different materials and determined that each one was different. This idea of mass and volume can tell us something about particles and mass. So we gave it a name… Density.