The mouth waters

from The Collaborative International Dictionary of English v.0.48

Water \Wa"ter\, v. i.
   1. To shed, secrete, or fill with, water or liquid matter;
      as, his eyes began to water.
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            If thine eyes can water for his death. --Shak.
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   2. To get or take in water; as, the ship put into port to
      water.
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   {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.
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