Most mammals have two sets of teeth just as humans have.
The whalebone whale has two sets of teeth, but both sets are lost before the baby whale is born. Neither set is ever used. Farmers in the United States, which has a highly mechanized agriculture, constitute less than ten percent of the population. In youth, most people maintain that black is black, white is white, and grey is nonexistent. It is only gradually that experience teaches that just as there is no absolute evil, so there is no absolute good.