A grain of salt

[1] In the old-fashioned English units of weight, a grain weighs approximately 65 mg, which is about how much table salt a person might pick up between the fingers as a pinch.

The phrase is thought to come from Pliny the Elder's Naturalis Historia, regarding the discovery of a recipe written by the Pontic king Mithridates to make someone immune to poison.

The phrase cum grano salis ("with a grain of salt") is not what Pliny wrote.

Pliny's actual words were addito salis grano ("after having added a grain of salt").

The Latin word sal (salis is the genitive) means both "salt" and "wit", thus the Latin phrase cum grano salis could be translated to either "with a grain of salt" or "with a grain of wit", actually to "with caution"/cautiously.