"Mami, mi pito esta duro."

With the advent of "learning Nick," I'm assured to be faced with a cornucopia of random questions and unfamiliar situations however I didn't expect this bomb.

This morning, as I rubbed my eyes in unlit surroundings, I was greeted by Nick who was holding open his underoos, staring in and requesting that I do the same.

"Mira lo que paso. Mami, mi pito esta duro. Porque?" (with a tilted headed, questioning glance and wispy hair covering his eyes)

Holding back what would wave been a fabulous bout of laughter I replied "Es normal. You just need to take a shower."

"Estoy sucio?"

(Fighting back additional laughter) "No. No. Pero vamos a banarte."

And off he went.

Here's the funny in parenting right? But alas, not having a penis of my own, I am unfamiliar with the circumstances and was truly not prepared for this question.
What the hell?
He is 3, almost 4, going on 30 sometimes- are penis questions supposed to be coming this early?

And how am I supposed to keep a straight face for that?