The reason is plain.

Those stones are rubber; therefore they are not stones.

Those rocks around us are rubber, and therefore they are not rocks.

Even this path is not a path; it's rubber.

Unless we are very careful, your Majesty, we are all likely to get the bounce, just as your poor officers and Tik-Tok did." "Then let's be careful," remarked Files, who was full of wisdom; but Polychrome wanted to test the quality of the rubber, so she began dancing.