oxytocin
Explanation
The Joke
A man at a bar is lecturing his companion about how people oversimplify the role of hormones, comparing it to expecting a single musical tone to only work in one kind of music. He then launches into a series of facts about oxytocin: it is associated with love and affection, but also with rapidly sobering up; it is associated with childbirth, but you can also release it by manually stimulating your nipples. His companion, now visibly disturbed, asks "So... right now you're NOT trying to be sexy, or..." The man angrily replies, "I am preparing to drive, you creep!"
The comic subverts expectations by setting up what seems like a pretentious bar lecture about neuroscience, then revealing that the speaker has been stimulating his own nipples at the bar -- not for any romantic reason, but because he wants to sober up faster to drive home safely. The "educational" framing makes the grotesque physical act even funnier.
The Humor
The comedy hinges on the gap between the intellectual, professorial tone of the lecture and the deeply weird physical reality of what the speaker is actually doing. The listener's discomfort mirrors the audience's own reaction. The final punchline lands because the speaker is genuinely indignant -- he considers himself the reasonable one, using science-based methods to be a responsible driver, while his companion is the "creep" for assuming anything sexual. It is a classic SMBC move of taking a real scientific fact and following it to its most socially awkward logical conclusion.
References
Oxytocin is a real hormone involved in social bonding, childbirth, and lactation. While nipple stimulation does trigger oxytocin release, the claim about it helping with sobering up is a humorous exaggeration of its complex physiological roles.