Salt Grains

I've become overly literal of late. In the process I'm finding it difficult to understand the origins of some of our more common cliches. For instance, take it with a grain of salt.

When someone outright lies to me, makes crap up, or says something just plain stupid, how exactly does this grain of salt we like to mention help? Sure it's simply a polite way of saying someone I have to put up with, since they're family or a friend of a friend, is an idiot and says dumb shit so just ignore him. Where I get lost is how salt entered into the picture?

Update
This is why I hate the internet. I miss not knowing stuff. At this rate I'll be able to answer trivia questions, start enjoying jeopardy, and actually get one of those pie pieces when someone forces me to sit for a game of trivial pursuit:

"Salt was thought to have healing properties and to be an antidote to poisons. To take (eat or drink) something "with a grain of salt" was to practice preventive medicine. One would do this if they were suspicious that the food might be poisonous or may cause illness."

It's going to take me weeks to forget this and return to my blissful saline ignorance.