What is hidden in 3D image?

Two old drunks are sitting in a bar when the first one says, "Ya know, when I was thirty and got an erection, I couldn't bend it, even using both hands.
By the time I was forty, I could bend it about ten degrees if I tried really hard.
By the time I was fifty, I could bend it about forty five degrees, no problem.
I'm gonna be sixty next week, and now I can bend it in half with just one hand."
"So," says the second drunk, "what's your point?"
"Well, I'm just wondering how much stronger I'm gonna get."