Not nail side, the other side. He takes quite a few layers off until they bleed and then tells me he tripped over. I know he is biting them though because i've seen him.
I've asked him why he is doing it and told him to stop, but he said he doesn't know why and he can't stop. He said it does hurt though.
I can't really put anything foul tasting on them because it would sting too much.