It's a Common Problem

Whenever I sense the word "its" or "it's" coming up in a sentence I read, I hold my breath a little. I'm waiting to see if the apostrophe will appear and if it's supposed to be there. This is one of the most common mistakes I see, and it's a mistake that's easily avoided.

It's simple. To show that something belongs to the subject, use "its." (The dog wagged its tail.)
If you can use "it is" instead, use "it's." (See above: It's simple.)

A restaurant I used to go to proudly stated on its menu: "Pasta at it's finest." I wonder if I could have fixed all the menus if I had eaten there often enough. For the record, I didn't fix any of them...yet.