What's the diff? “it’s” & “its”

When should one write "it's" vs "its"? I memorized the rule some years ago, but today I noticed for the first time a parallel that should help: its::his=it's::he's

So if you can remember when to use "his" vs "he's" then you're all set:

• He's a wise dog : he scratches his own fleas.
  
• It's a wise dog that scratches its own fleas.
  
• Note that the underlined green text (he's, it's) mean "he is" or "it is".
  
• The yellow italic text (his, its) mean "belonging to (him or it)"

Not many things in American English are this consistent, so it's nice to know that the use of each word is consistent with that of its partner.