You didn't know that, did you?
Yup, my parents taught me when I was a kid that it was okay to hold your middle finger up as long as you weren't thinking bad thoughts.
I'm pretty sure I caught one of my parents or their friends giving someone the finger, knew it was a bad thing to do -- though didn't know why -- and they explained it away with the above. Seemed to make sense to me at the time. Oh, the poor fools who didn't know!
Imagine my confidence in my parochial school lunchroom in 3rd grade as I waved my middle finger at my classmates and declared to their horrified faces that it was okay because, "I'm not thinking bad thoughts!" The kids didn't seem to agree with this interpretation, nor did my teachers. Unenlightened lemmings!
So the next time someone flips you off, why don't you give them a friendly wave in response? And be sure to yell out to them, "It's okay! I know you're not thinking bad thoughts!" And if they raise their other middle finger to you as well, be sure to forgive them again. Hopefully by then they'll think you're insane and just leave you alone. :)