Hey, vsauce, Micheal here!
Why is my peepee hard? Or is it?
Well, ya see, we need to start with the peepee itself. The hardening of the peepee is called an erection.
Erection.
The term itself shares a similarity with our own predecessors, Homo Erectus. That's Latin for Gay Straight. So, naturally, we can start to fill in the picture. Our predecessors, Gay Straights, also had hard peepees. Which brings us back to modern day erections. You see, an erection is a signal to tell you that you are gay, a leftover device from the era of Gay Straights.
But what is "gay"?
To quote Nelson Mandela, "To be gay is not to love others. It is to love yourself and to stay true in the face of many hardships and hard dicks." In Layman's terms, Nelson Mandela is trying to tell us to go fuck ourselves. This is vital to know because it puts into perspective the absolute size of gayness itself, divided into unique homogenous subgroups. Such a diverse range is exactly why the flag of LGBTQBBQ communities is a rainbow.
But, you ask, how does this tie in to what gay Is? I'm getting there. Gay is a slang term for homosexual, which is to have sexual interest in someone of the same gender as yourself. To see more on the topic of gender, click the annotation on my face now for a playlist of videos specifically on the subject. So, to answer your question, your peepee is hard because you're gay.
Hopefully today's little snippet was a good brain train for you. If you'd like to find other fun ways to test your mind mettle, click the annotation up here to go see some of my huge DONGs. And, as always, thanks for watching.
you are now manually breathing 
HEARTHSTONE IS UNFAIR! 
He could be a Calvin Klein model.