My daughter did this also when she was about 20 months. We figured out she only did this when she was frustrated or angry about something. The only thing that helped throughout this period was giving her a small teething ring. I told her that when she was angry to bite the teething ring. This gave everyone (us and her daycare provider) the heads up she was angry. Though it did not totally stop the biting, it did helped her until she could control the urge to bite. She is 2 1/2 now and doesn't have any more biting episodes. Hope this helps a little.