It's and Its

The proper use of it's and its seems to be a common problem for people of all ages, myself included. I'm sure that at some point we were all taught the proper usage but for those like myself who have forgotten here is the rule:
It's is a contration of the two words it and is, while Its is a possesive pronoun.
The website I found for this answer suggested that in times of uncertainty all we have to do is replace the word in question with it is, and if that does not sound right then it probably is not.

http://www.copyblogger.com/5-common-mistakes-that-make-you-look-dumb/