Why does my new pet rat try to nibble my fingers?

I have recently bought a pet rat, and when i tried to pick her up, she sort of tried to nibble/bite me, but didn't do it hard enough to break the skin or even hurt, and she did it quite slowly. Today i put my hand in with some food on my fingers and she did it again. I'm worried that this could turn to biting, and am getting wary of her. Why does she do this?