The dentist uses special medicine called anesthetics to numb your mouth if you have a cavity taken out or a tooth pulled.
The first thing the dentist does is place a small cotton swab dipped in medicine in your mouth, and rubs in the area he needs to operate on. This medicine is called a "local anesthetic," and it numbs the surface of your mouth, or your gums. Sometimes, the dentist may need to give you a shot of more powerful medicine to really put your mouth to sleep. But the shot only stings a little bit because of the other medicine he used on the cotton swab!
This more powerful medicine is usually called Novocain or Lidocaine. The medicine is injected into the inside of your mouth using a very small needle. After a few minutes, a part of your mouth gets tingly, and then numb. Some people feel like they have a fat lip when their mouth has been numbed. Don't worry. Your lip doesn't get fat or swell. This goes away after a while.