The blocks should look as nearly identical as you can make them. Ask them to describe the blocks, making sure that they confirm that the two blocks are the same size. Pass the blocks around, asking the children to not comment on them as they pass them to each other. Once everybody has handled both blocks, discuss what was different about them. Explain the concept of density to them, and then discuss which of the blocks is more dense.
Density equals mass divided by volume.
If you divide the volume of the brick into its mass, you will get a much higher figure than by dividing the same volume of Styrofoam into its mass. The brick has much more material packed together into the same amount of space than the foam. The foam is composed mostly of air bubbles, which makes the Styrofoam extremely light. You could take a large piece of Styrofoam and weigh it. Then find a stone that weighs about the same amount, in order to show the reverse factors of the above experiment.