Good answer

I just put Amiri to bed.  Even though he's a big underwear-wearing boy, we ask him to wear a pull-up at night because sometimes it's difficult to make it to the bathroom when you're sleepy.  So as usual, tonight he got changed, and then began to carefully arrange his underwear on his bike. 

"Why are you doing that, Amiri?"

"Why not?"

Delivered so offhand and nonchalantly, I couldn't argue with that.  The bike will wear underwear tonight.