life's hard lessons

for most of us, life has dealt us at least a couple of hard lessons. do this, this happens,and it's not good. there are a few annointed ones that have yet to experience this, but they are few and far between.
as we age, our lessons tend to become harder, and extract a heavy price for the newfound knowledge. but the lessons begin young.
we were talking about this, and the question came up: "What is the first hard lesson you remember?"
one of my coworkers: "Never use tweezers to get something out of a light socket"
mine: "Don't lick the goodies off of can lids". i imagine that if we were using tongue prints rather than finger prints, my distinguishing characteristic would be a scar line across my tongue.
how about you?

edit: another from a coworker: "Staple guns do not have to be pressed against something to get them to fire a staple. And 2 inches away isn't far enough to NOT stick"