Usher's madness begins to increase because he can hear the sound of Madeline below.
Even though his sister is buried in the basement, he can constantly hear her scratching her coffin. He was tortured by these sounds until the protagonist came to visit him so he could share his troubles with him. Usher is afraid of his 'undead' sister and of dying himself, which is why he grows crazier and crazier by the day, until he is finally killed by his own sister in the end.
It means that teeth are very helpful because they help you chew your food so therefore we can digest it and maintain good health
