Why Do They Do That?
Answers to Common Questions About Cat Behavior
"Nibbling"

My cat nibbles or sucks on my ear when I’m lying in bed. Is he trying to hurt me?

          No. This is a grooming behavior that indicates your cat is happy, healthy, and has accepted you as its preferred companion. Cats who live together, and are comfortable with each other, will often nibble each others’ ears, tails, and fur to help keep each other clean. It’s especially common in cats who were removed from their mothers too early. There’s nothing dangerous or threatening about it.

          Most cats realize that human skin is much thinner and more delicate than their own, and will take care not to nibble too hard. If your cat does nip too hard, gently grasp it by the back of its neck (this is called "scruffing") and tell it "No!" in a firm voice.

          Scruffing is a very safe and natural way to train a cat, because when a kitten misbehaves, its mother scruffs it and carries it that way until it behaves. Scruffing your cat will reinforce the myth (believed by humans, though not by cats) that you are the boss, and must be obeyed. It won’t take long for your cat to learn to nibble and groom you gently.