Grain of Salt
From OS X dictionary
Grain of salt
Pliny the Elder’s Naturalis Historia may be the origin of the phrase.
To take something with a “grain of salt” or “pinch of salt” is an English language idiom that means to view something with skepticism or not to interpret something literally.[1] In the old-fashioned English units of weight, a grain weighs approximately 60 mg, which is about how much table salt a person might pick up between the fingers as a pinch.