What is "the mouth waters"

Water \Wa"ter\, v. i.

1. To shed, secrete, or fill with, water or liquid matter; as, his eyes began to water.

   If thine eyes can water for his death.
   --Shak.

2. To get or take in water; as, the ship put into port to water.

   The mouth waters, a phrase denoting that a person or animal has a longing desire for something, since the sight of food often causes one who is hungry to have an increased flow of saliva.