You need two sheets of sandpaper for this activity.

Look carefully at the surface of the sandpaper using a hand lens. It has grains of sand stuck to it, but between the sand grains there are pits.

Now place one sheet on a desk face up and the other sheet on top of it face down. Try moving the sandpaper sheets over one another.

Press down on the upper sandpaper sheet and try again while you continue to press down. What do you notice?