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@grammarnazi is right; this is an exception to that rule. "It's" means "it is".

To add insult to injury, I'm going to pile on (just in good fun) by pointing out that you should have used a semicolon, not a comma, and "Stiff upper lip" is a sentence fragment that doesn't quite cut it because too many words ("You must have a") are implied.

It's fun to be a jerk once in a while. I'm sure that you'll repay me if I give you the chance!

Yep, it's is a contraction, but as i am refering to the it having having something the apostrophie is appropriate.
It's the dog's bowl.
Doing the right thing is the right thing's own reward.
Here doing the right thing is shortened to it, ergo the apostophie's showing the it's possession.

This is why i hated skool, too many conflicting rools.

I forgot to mention how much I enjoyed your post. (A lot.) er... (Alot.) ???? He who has the best grammar checker wins.

I give exactly 0 fucks about your grammar? I'm more interested in understanding what you're trying to say! Grammar Nazis make me imagine radioactive aids blood pouring from their anuses!

Bunch of rool followers,...