Dear Fatima,
Biting nails is a habit which is not easy to quit. You have to stand firm in order to leave this habit. Here are some reasons and tips to avoid it.
•    Eat calcium and magnesium - rich foods so that your nails will repair and grow well. Not only that, but most of the reason that the human body wants to bite nails is the lack of calcium and magnesium in the body and the body needs that material back.
•    Distract your mouth. Eat carrot sticks to keep you busy. Keep a stick of gum handy for those weak moments.
•    Cover your nails. For females, fit artificial nails. They can really help. For guys, polish them and apply some shine/growth promoter or petroleum jelly. It's harder to bite nails that are looking great!
•    If you have any surplus growth, cut your nails back. Keep clippers with you at all times. You can't bite if there's nothing there!
•    Consult your physician.