Why Is She Biting?

So, I haven't written anything for the last couple of days. I have really been working on getting the kinks out of the website I am working on. It really isn't that easy when you basically don't have a clue what you are doing!
Today, Morgan (my 3 year old), bit her little sister. I didn't know for sure what had happened--I heard Amy cry, but Morgan wouldn't tell me what she had done. I figured it out about 10 minutes later when I saw the bite marks on Amy's arm. I have been so upset about it! I really had hoped that I was past the age of having to worry about the whole biting thing. I know some other people who have had trouble with their kids biting in daycare, so I hoped that Morgan wouldn't have had the opportunity to see it happening and maybe wouldn't start herself. Guess I was wrong there! I did ask her later why she did it, and where she had seen it. All I got was that "that's what you do when the boys are being bad." So, the only thing I can think of is the other day she was playing with some boys in the McDonald's Playland. Maybe one of the boys bit another--I'll probably never know. I guess you can't sheild them from life though. It is an experience you have to just help her through, and hope no other kids get hurt in the meantime! But, I gotta tell ya--Mommy was SO upset tonight! Any tips?