"it's" = "it is" or "it has"

About five years ago, my good friend, then boss, and English major Michael Yoon taught me this very simple rule: “It’s = it is” or “it has”. If you do not intend to say “it is” or “it has”, you’ll want to say “its”. Since finally learning this simple and hard rule, my eyes have been hurting every time I’ve seen this mistake made. (The final straw)