Self explanatory, really, just tell a lesson, silly or more serious, when you were little.
I learned a very important lesson about the importance of underwear once when I was nine.
When I got up that morning, I couldn't find any pairs of underwear for some reason in my underwear drawer, so I just decided: "Screw it, I just won't bother!" So I put on my favorite pair of sweatpants and went on about my day…without realizing that there was a tiny hole in the front.
It wasn't until that afternoon when I was playing a game with my siblings that I realized I needed underwear. Everything was going normally, until they both gasped and pointed toward my lower half. I looked down to find that, somehow, a certain little "thing" had managed to be sticking out of the little hole in the front of my pants. I promptly went and put on underwear.
The lesson here is either make sure you put on your undies, or make sure your pants don't have holes in them. O_o