From Hyperbole and a Half:
At some point during my childhood, my mother made the mistake of taking me to see an orthodontist. It was discovered that I had a rogue tooth that was growing sideways.
My mom and I were told that the tooth, if left unchecked, would completely ruin everything in my life and turn me into a horrible, horrible mutant.
Unless I wanted to spend the rest of my natural life chained in a windowless shed to avoid traumatizing the other citizens, I was going to need surgery to remove the tooth.
I was accepting of the idea until I found out that my surgery was scheduled on the same day as my friend's birthday party. My surgery was in the morning and the birthday party wasn't until the late afternoon, but my mom told me that I still probably wouldn't be able to go because I'd need time to recover from my surgery. I asked her if I could go to the party if I was feeling okay. She said yes, but told me that I probably wouldn't be feeling well and to try not to get my hopes up.
But it was too late. I knew that if I could trick my mom into believing that I was feeling okay after my surgery, she'd let me go to my friend's birthday party. All I had to do was find a way to prove that I was completely recovered and ready to party. I began to gather very specific information about the kinds of things that would convince my mom that the surgery had absolutely no effect on me.
More here. [Thanks to Jennifer Oullette.]