Among the jokes classified by Ted Cohen as hermetic in Jokes: Philosophical Thoughts on Joking Matters (University of Chicago Press, 1999) are the following that he classifies as strongly hermetic:
What did Lesniewski say to Lukasiewicz? "Logically, we're poles apart."
What is a goy? A goy is a person who is a girl if examined at any time up to an including t, and a boy if examined at any time after t.
One day a paleographer came into his classics department in great excitement. "There has been an earth-shaking discovery," he anounced. "The Illiad and the Odyssey were not written by Homer, but by some other Greek with the same name."
If you got those, then try this severely hermetic one on for size:
What's round and purple, and commutes to work? An Abelian grape.
These three also fall under the hermetic rubric, though they are not especially so:
According to Freud, what comes between fear and sex? Fuenf.
A young Catholic woman told her friend, "I told my husband to buy all the Viagra he can find." Her Jewish friend replied, "I told my husband to buy all the stock in Pfizer he can find."
After knowing one another for a long time, three clergymen — one Catholic, one Jewish, and one Episcopalian — have become good friends. When they are together one day, the Catholic priest is in a sober, reflective mood, and he says, "I'd like to confess to you that although I have done my best to keep my faith, I have occasionally lapsed, and even since my seminary days I have, not often, but sometimes, succumbed and sought carnal knowledge." "Ah well," says the rabbi, "It is good to admit these things, and so I will tell you that, not often, but sometimes, I break the dietary laws and eat forbidden food." At this the Episcopalian priest, his face reddening, says, "If only I has so little to be ashamed of. You know, only last week I caught myself eating a main course with my salad fork."