Right now I'm reading a book titled Eats, Shoots & Leaves: The Zero Tolerance Approach to Punctuation, by Lynne Truss.

I simply must share a passage from this marvelous literary masterpiece (found on pages 43 and 44):

"To those who care about punctuation, a sentence such as "Thank God its Friday" (without the apostrophe) rouses feelings not only of despair but of violence. The confusion of the possessive "its" (no apostrophe) with the contractive "it's" (with apostrophe) is an unequivocal signal of illiteracy and sets off a simple Pavlovian "kill" response in the average stickler. The rule is: the "it's" (with apostrophe) stands for "it is" or "it has". If the word does not stand for "it is" or "it has" then what you require is "its". This is extremely easy to grasp. Getting your itses mixed up is the greatest solecism in the world of punctuation. No matter that you have a PhD and have read all of Henry James twice. If you still persist in writing, "Good food at it's best", you deserve to be struck by lightning, hacked up on the spot and buried in an unmarked grave."

Amen and AMEN!!