When the great library of Alexandria burned, the story goes, one book was saved. But it was not a valuable book; and so a poor man, who could read a little, bought it for a few coppers.
The book wasn't very interesting, but between its pages there was something very interesting indeed. It was a thin strip of vellum on which was written the secret of the "Touchstone"!
The touchstone was a small pebble that could turn any common metal into pure gold. The writing explained that it was lying among thousands and thousands of other pebbles that looked exactly like it. But the secret was this: The real stone would feel warm, while ordinary pebbles are cold.