On my 16th birthday, my mom joked that I was too shy to be naked in front of her now(I had gotten old enough that It really was quite humiliating if she saw my Willy). I jokingly said I wasn't shy at all, and she dared me to "show her my birthday suit" if I dared.
I just turned around and dropped my pants and undies till my knees ( to moon her as a joke) saying "This is all I'll show you
" but and she started laughing. I thought she was just laughing at my antics of showing my bare butt , until 30 seconds later when she finally stopped laughing and said "If you were going to show me just your ass, you shouldn't do it in front of a glass cupboard, I saw all your goodies 
"I hurriedly pulled up my pants but it was far too late, she'd been watching it for over half a minute at that point and she mercilessly teased me about the fact she saw my pee pee because of my own stupidity.
