Rats don't bite unless they are ruby-eyed or pink-eyed rats, as for they cannot see as well as their black-eyed cousins, and will think your finger is a treat. This rarely happens. When a rat NIPS you, that doesn't mean aggression. It means they want to play, or, again, they think you have a treat. If it's a serious bite, you probably scared your rat, or your rat might not be used to you yet, and is nervous and thinks your hand is a predator.