At one time or other, each sister and brother
Has pondered the musical question
(The topic’s not easy, just take it from PZ):
How an eyeball is like an erection.

The answers may vary—be skeptically wary—
Like “Both can display your affection.”
Well, so can a rose, but that doesn’t disclose
How an eyeball is like an erection.

Perhaps evolution provides a solution
Both organs arise through selection
But so, then, do fingers; the question still lingers
How an eyeball is like an erection.

We may hope to deduce, if we try to reduce
To a chemical sort of connection
But will “similar stuff” prove an answer enough
How an eyeball is like an erection?

Nitric Oxide (you know, you can call it NO)
Causes GuMP to take up a collection
So that GuMP, for a lark, keeps your dick “in the dark”
Thus an eyeball is like an erection

Reproductive success got us into this mess
So it might get us out, on reflection—
But Viagra, we find, is not blindly designed
We distinguish both eye and erection.

With both vision and hearing, the answers are nearing
(Although we can’t hope for perfection),
And for now it’s just fine as a bad pick-up line:
How an eyeball is like an erection.*

(*answer: “It’s an empirical question—let’s experiment, and find out.)

Pharyngula asks the question…
…based on effectmeasure’s post.