HELLLLLP!!

OK.....today at daycare my son decided to bite a little boy on the arm, while they were playing. Colby bit Silas so hard that it left teeth marks in his arm and I could still see the marks at 5:00pm when we left....he did it at like 10 this morning. Silas is a very pale, white child but still he must have done it pretty hard to leave marks like that.....I was just sick about it! Silas is not a very aggressive child so I have no idea what went on, but the biting has to STOP!! Help.....any ideas on how to get Colby to STOP biting! He was just biting Aaron or I when he was mad, but that stopped about 5 months ago, but today....WOW, I don't know what came over him! SO if you have any ideas, please help!! I'll keep you posted on the biting!! UGH!