When DD was three:
"We don't put chapstick on our vulva. It doesn't need chapstick".
Now the worst part. I re-cap the chapstick and set it on the nightstand as I re-direct DD to another part of the house. Yep, I forgot about it and an hour later DH comes walking into the kitchen applying a generous amount of chapstick to his lips.
He didn't find it nearly as hilarious as I did!