I had those already written, long enough ago that I can't even remember the occasion, though I suspect it was online banter somewhere. Anyway, here's a lesser, newly minted one that does not restrict itself to a single sentence:
The Professor intoned, "The average person would not drink his own urine, but willingly sniffles his own post-nasal drip, a substance far more vile." A young woman taking notes in the front row suddenly sneezed so violently that the outpouring splattered the blackboard to either side of the Professor and dirtied the previously immaculate lens of his circular pince-nez. From the back of the room, a derisive voice called out, "Teacher's pet!" The Professor's heart skipped a beat as he glimpsed the woman, as if for the first time, through his newly viscous spectacles.