Dental drill on tooth. Coloured scanning electron micrograph (SEM) of a diamond-tipped dental drill (orange) drilling a hole into a tooth (blue). The drill's rough diamond tip (lighter orange) is extremely hard. Drills are used to remove decayed or damaged areas of tooth in preparation for dental fillings. Decay, or dental caries, is caused by plaque, a sticky mixture of food particles, bacteria and saliva that builds up on teeth. The bacteria in plaque feed on sugars in the food particles, and produce an acid by- product that slowly destroys the tooth. Magnification: x10 at 6x7cm size.