You want to know why I am so fond of that little bit of limestone, no bigger than my hand, which lies upon the shelf; why I ponder over it so often, and show it to all sensible people who come to see me?
I do so, not only for the sake of the person who gave it to me, but because there is written on it a letter out of Madam How's alphabet, which has taken wise men many a year to decipher.

I could not decipher that letter when first I saw the stone.

More shame for me, for I had seen it often before, and understood it well enough, in many another page of Madam How's great book.

Take the stone, and see if you can find out anything strange about it.
Well, it is only a bit of marble as big as my hand, that looks as if it had been, and really has been, broken off by a hammer.

But when you look again, you see there is a smooth scraped part on one edge, that seems to have been rubbed against a stone.
Now look at that rubbed part, and tell me how it was done.
You have seen men often polish one stone on another, or scour floors with a Bath brick, and you will guess at first that this was polished so: but if it had been, then the rubbed place would have been flat: but if you put your fingers over it, you will find that it is not flat.