That finger is a treat!

Pet Rat Information Sheet - all you need for happy healthy pet rats: "Finger nipping may occur if your rats are used to getting treats through the cage bars. This is not true biting, but merely an accidental nibble. If a finger is poked through the bars too, the rats may nip, mistaking the finger for food. Train your rats to tell the difference, by telling them when food is arriving - eg 'Sweeties!' - or fingers, eg 'Be gentle!'. If this fails, stop feeding treats through the bars; instead, open the cage door to put your hand inside when hand-feeding."

Watch out for your fingers...