By the age of 18, the average adult has 32 teeth; 16 teeth on the top and 16 teeth on the bottom. The back teeth (molar teeth) are used to grind food up into a consistency suitable for swallowing. The average mouth is made to hold only 28 teeth. It can be painful when 32 teeth try to fit in a mouth that holds only 28 teeth. These four other teeth are your third molars, also known as "wisdom teeth."