The rule is: the word '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.
Lynne TrussNo one else understands us 7th sense people. They regard us as freaks. When we point out illiterate mistakes, we are often aggressively instructed to 'get a life' by people who, interestingly, display no evidence of having lives themselves.
Lynne TrussTexting is a fundamentally sneaky form of communication, which we should despise, but it is such a boon we don't care. We are all sneaks now.
Lynne TrussPunctuation is a courtesy designed to help readers to understand a story without stumbling.
Lynne Truss