Biting is a common behaviour of closing the jaw intending to harm something found in many animals (including humans), often as part of attacking others, eating or sexual arousal. Some discussion of human biting appears in The Kinsey Report on Sexual Behavior in the Human Female.
Biting is also an age appropriate behavior and reaction for human children 2.5 years and younger. Conversely children above this age have verbal skills to explain their needs and dislikes and biting is not age appropriate. Biting may be prevented by methods including redirection, changing the environment and responding to biting by talking about appropriate ways to express anger and frustration. School age children, those older than 2.5 years, who habitually bite may require professional help. 
- Child Care Links, "How to Handle Biting", retrieved 14 August 2007