1. You bite your nails--It's one thing if you nervously bite your nails only during scary movies, but when it becomes a regular habit, it can damage both your nails and the skin around them. Germs from the mouth get transferred to the skin, and vice versa. Try tape to break the habit.

2. You twirl and pull your hair--Twisting and twirling a piece of hair around your finger can lead to damage to the root over time. That can lead to temporary or permanent areas of hair loss as well as infection. Obsessive hair pulling may be a sign of a psychiatric impulse control condition called trichotillomania, which requires psychotherapy and medication.

3. You crack your neck--Twisting your head forcibly to one side releases gases built up in the joints between vertebrae and creates a popping sound. That may feel good, but repeatedly cracking your neck can make the ligaments more susceptible to injury. This motion can also cause wear within the joints and can result in arthritis over time.

4. You touch your face--Repeatedly touching your face or picking at acne can damage the top very thin microscopic layers of the skin. If you bleed, you may create a permanent scar. Don't pick at pimples or itchy areas. Treat them gently with topical creams and plenty of moisturizer.

5. You grind your teeth--Clenching and grinding your teeth when you're under stress can wreak havoc on your oral health. Grinding can cause teeth to crack or break, which may require repair with crowns or root canals. It can also result in damage to the jaw joint. Treatments include orthodontics to improve the bite and even Botox injections in the muscles, which can reduce the amount of force and the potential damage.

6. You suck on hard candies--Sucking on hard candies bathes your teeth in sugar, which can lead to cavities. Bacteria feed off the sugar, which creates a perfect environment for tooth decay. Chomping down on hard candy can also risk damaging teeth. Sucking on candies in moderation is fine as long as the candies are sugarless and low in acidity.

7. You lick or bite your lip--Nervously licking your lips exposes them to your mouth's digestive enzymes. These enzymes chew away at the skin and can lead to inflammation, which make lips appear dry and cracked. Biting your lips when under stress can cause the development of fibromas that may require surgical removal.

8. You gnaw on the inside of your cheek--Like biting your nails, cheek-chewing can also become a nervous habit. The inside of the cheek gets swollen and it becomes easier to continue biting the same spot. Even after it heals the habit continues. Over time this can cause chronic inflammation, possible bleeding, and scarring of the area.

9. You chew gum--All that snapping and popping can put you at risk for TMJ from overuse of jaw muscles. Sugarless gum has a different set of problems, mostly digestive. Sorbitol is an artificial sweetener that produces an unpleasant laxative effect when eaten in excess (18 to 20 sticks a day). Swallowing excess air while chewing also increases risk of a gassy stomach.

10. You nibble the ends of pencils and pens--Germs can hang out on the ends of pens so this habit can expose you to nasty ones including cold viruses. Chewing on writing instruments can also damage teeth and dental work as well as injure the soft tissue and gums inside the mouth.