Don't Sound (or Write) Stupid, Part 3

A new feature wherein I, Lydia, instruct you, the reader, not to make common, idiotic mistakes of grammar, syntax, or pronunciation of the English language, particularly errors that annoy me. As a review, the word "recognize" is pronounced rek-kog-nize, not reckin-nize.

Special request: I have been asked to explain the difference between its and it's. This is a difficult one and the bane of even educated writers. Somehow the mists of time or convoluted explanations have blurred our understanding, leaving everyone guessing (and often guessing wrong.) 

Actually, this is very easy. 

It's is always a contraction for "it is" or "it has." If you cannot substitute either one, use "its."