A personal story: In middle school, I was a chubby kid and my sport teacher made fun of me telling me I could not run a 5 miles contest because my heart may stop beating. I told my dad, my best friend, begging him not to complain at school. I saw the anger on his face and he told me one thing: prove him wrong son! I started running every day and lost tons of weight.
At my last year of high school the teacher was still around. One day I challenge him in front of a bunch of student and some teachers to run the five miles against him. He was dumb to accept because he failed to realize that I was no more the chubby kid, I was thinner and about 4 inches taller than him. I ran the first 4 miles right behind him and past him at the last mile. I remember the humiliation he put me in years back and called him names on the last stretch in front of everyone. He was crushed; I had my revenge and still feel good about it today.