A.M.
My daughter is 3 and did that for a short while. Whenever she started doing it I would very firmly tell her "STOP". It would kind of shock her out of it, and I would then tell her she was going to give her teeth owies. Told her she would put holes in her teeth. Well my daughter is very into her appearance (check out some of my other posts) and is really good about taking care of her teeth, so this worked. She doesn't want owies or yucky teeth, so she stopped after awhile. Good luck!