"Oh, shoot, I'm sorry, I have my period and must have had an accident. I'll clean it up."
He looks at me, my baffled 10 year old and says, "you bleed when you have your period? I thought you were just super grumpy."
I'd had the talk about sex and reproduction with Joe, in fact, it's not been a taboo subject in my household at all (as illustrated by my response to his disgust), so I looked back at him - just as baffled - and said "let me explain it again."
I drew the ovaries and the fallopian tubes and the uterus and the vagina and explained how every month an egg is released and travels to the uterus and if it's not fertilized by a sperm it needs to come out an that is a period. Every woman has one. "Some of your friends at school probably already have it. I got mine when I was 9 ...."
"So are you saying that Maia will get her period someday?" he asked.
"Yes." Someday his baby sister would, indeed, get her period.
He responded, "So you're saying you will BOTH have your periods and you will BOTH be super grumpy every month?"
And that is how I knew my boy had become a man.