"Take it with a grain of salt" is an idiom. An idiom by its very nature has a meaning different from its literal meaning. You'll misunderstand an idiom if you try to reason about it from the literal meaning of the words.
A common example is "I could care less." When you see that phrase in a forum of literal-minded people like this one, someone is likely to reply that they should have said "I couldn't care less." Yes, of course that is what the writer meant - but this doesn't make the idiom wrong. The idiom is sarcasm - saying the opposite of its literal meaning. This is more clear when you hear it spoken out loud: the emphasis is on the words "I" and "care", indicating the sarcastic intent.
Or take the phrase "kicked the bucket", which obviously does not literally mean "died". In the movie It's a Mad Mad Mad Mad World, when Jimmy Durante's character dies after his car flies off the road, there happens to be a bucket next to his foot and in his death spasm he kicks it. What makes the scene funny is the use of the idiom's literal meaning - unlike the normal use of an idiom.
In fact, you might say they turned the idiom on its head. But idioms don't have heads, do they? :-)
A common example is "I could care less." When you see that phrase in a forum of literal-minded people like this one, someone is likely to reply that they should have said "I couldn't care less." Yes, of course that is what the writer meant - but this doesn't make the idiom wrong. The idiom is sarcasm - saying the opposite of its literal meaning. This is more clear when you hear it spoken out loud: the emphasis is on the words "I" and "care", indicating the sarcastic intent.
Or take the phrase "kicked the bucket", which obviously does not literally mean "died". In the movie It's a Mad Mad Mad Mad World, when Jimmy Durante's character dies after his car flies off the road, there happens to be a bucket next to his foot and in his death spasm he kicks it. What makes the scene funny is the use of the idiom's literal meaning - unlike the normal use of an idiom.
In fact, you might say they turned the idiom on its head. But idioms don't have heads, do they? :-)