According to Little Brother, it's "eenie, meenie, miney, mo."
This morning while I was getting dressed after my shower he urgently banged on the bathroom door. "Mom, I need the bathroom!"
"Can't you use the one downstairs?" I asked him.
As I let him in, I asked him why he had to run upstairs (when he started out downstairs) to use that bathroom.
"I did eenie, meenie, miney, mo. That's fair."
"But why did you have to do that when one bathroom was already used and the other one was empty?"
"Mom, Mom, Mom," he said, in that "she just doesn't get it" tone that I didn't think he'd adopt for a few more years. "I had to do it so it would be fair."
Because, after all, the bathroom might get offended if he didn't choose fairly.