The difference between your and you're

Ok, this one should be simple, yet it's a mistake I see WAY too often! 
If you can substitute 'You Are' then use You're.  If you can't then it's Your. 

Your is a possessive: 
Is that your dog?
Your eyes are pretty.
Your grammar is lacking.

You're is a contraction of You and Are:
You're so stupid
Do you know if you're going to the party?

It make take 2 extra seconds to think it out, but please take the time!  Otherwise you WILL be judged and mocked!