"It's a pun!" Lucy points out helpfully as Peter turns the rock over in his hands again. It's... well, it's a very nice rock to be sure, but when he looks up to ask Father Christmas if he's pulling his leg, the old man is already riding off in his sleigh.

Still, it gets the job done when he lobs it at the White Witch and clocks her in the temple, knocking her (heh) stone dead with one blow.