THE TEETH 231 



shaped mouth. The habit of constantly sucking in 

 the lower lip is harmful for a similar reason. Sucking 

 the thumb may be prevented by tying something 

 around it that will make it too large to be admitted 

 to the mouth. To stop lip sucking, doctors sometimes 

 insert a piece of rubber between the lips and teeth. 

 As we have learned in a previous chapter, the habit 

 of constant mouth breathing also causes a similarly 

 deformed appearance. It is now possible to have an 

 irregular set of teeth straightened by a dentist. Of 

 course this should be done as soon as possible after 

 the principal permanent teeth have come in. 



Thus far we have read about what to do to keep the 

 teeth clean and healthy. Let us now consider some 

 things not to do. Cracking nuts with our teeth' or bit- 

 ing any substance harder than food should be avoided. 

 Biting thread is an injurious habit which girls especially 

 need to guard against. The great force exerted to 

 bite the thread often injures the enamel of the under 

 teeth. Such use of our edged tools may break the 

 enamel and cause serious injury. Pins or any metallic 

 substances are likely to scratch the enamel and do 

 permanent damage. For this reason, picking the 

 teeth with any sharp or pointed metal is unwise. In 

 a previous chapter we learned that heat expands 

 and cold contracts air. Both have a similar effect 

 on the enamel of teeth. On this account, a hot food 

 or drink should not be immediately followed by one 

 that is cold, and vice versa. When expansion and con- 



