Ask students to imagine using a pencil eraser so much that the whole eraser becomes a pile of tiny eraser rubbings. If you saved every tiny piece of rubbing and piled them together, would they have the same weight as the original eraser? If each tiny rubbing had zero weight, how could there be any weight when you pile them all together? Where would that weight come from? Isn’t the eraser like a collection of eraser rubbings all melted together?