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.

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.

Panda. Large black-and-white bear-like mammal, native to China. Eats, shoots and leaves.

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 Pavlovian "kill" response in the average stickler.

When you by nature subscribe to the view that everyone except yourself is a berk or a wanker, it is hard to bond with anybody in any rational common cause.

The reason it's worth standing up for punctuation is not that it's an arbitrary system of notation known only to an over-sensitive elite who have attacks of the vapours when they see it misapplied. The reason to stand up for punctuation is that without it there is no reliable way of communicating meaning.

Punctuation marks are the traffic signals of language: they tell us to slow down, notice this, take a detour, and stop.