Not mine but a classic.
Friend of Samuel Beckett walking with him on a beautiful spring day: “A day like this makes you happy to be alive.”
Beckett: “Almost”
OK, that’s even better than “The unit vector antiparallel to the local apparent gravitational field”. I have a lot of respect for an herb which can out-nerd me gravitationally.
A couple of mine:
On returning from a vacation, a colleague says “You’re back!”. I look over my shoulder, and say "What about my back?
One day I was working on an assignment that was due a day or two earlier, and another student asked “Got a little behind, eh?”. I stood up, looked back, and said “No, I’d say it’s about the normal size.”.
Whoever, “How is your day going?”
Me, “Ok, but it is not over yet.”
That brings to mind the fake SNL news show when the announcer said, “Now, for a look back.” He then turned and looked behind him, faced the front again, and continued with the news report.
To my everlasting regret, I didn’t think of this response until later.
My father, who clings hopelessly to classical gender-role stereotypes, was driving my family and a couple of my friends around on vacation. We were admittedly lost, and my mother told my father to stop and ask someone for directions. My father says, “What kind of man would I be if I asked for directions?”
What I should have said: “A man who knows where he’s going.”
<Dave relates a wacky story to me>
Me: You’re crazy…
Dave: What?! Why am I crazy?
Me: …maybe you were born that way.
<At a Denny’s after a long night of drinking>
Waitress: … and how do you like your eggs?
Dave: IMPREGNATED!
(I left a big tip for that jackassery.)
herownself asked: “When did you get your haircut?”
Me: “You want a list?”
Me and friends, standing on the pier at midnight, hanging out for no reason. A woman walks by.
Us: “Nice night!”
Her: “Yes, much to the dismay of Satan’s people.”
Have you lived here all your life?
Not yet.
When asked “How are you?”, I often reply “Alive.”
It makes people think.
I’ve found that people don’t like to think…
Well I see that ‘a possum’ is an animal that lives in the woods.
But what is the pun meaning of ‘geometric figure’? 
FWIW, “opossum” is an (I think) Americanism for “possum”. I’ve always heard them called “possums” in the spoken language, but the word “opossum” would be recognized by most here. Hence “apothem” is “opossum”, not necessarily “a possum”. Just so’s ya know.
Alright, but I’m still not seeing what this has to do with William Shatner or a geometric figure.
My favorite:
Did you take a bath?
No, why- is one missing?
I believe Shatner voiced the character of an opposum in the movie in question, and there is a geometric thingy called an “apothem” (IIRC, it’s like a “radius”, only not of a circle), pronounced roughly like “opossum”, if one had a lisp.
WhyNot,
Murdering humor with explanations since 1974.
(It’s taking longer than she thought.)
OK…That was the missing piece of the puzzle.
On behalf of the rest of us who didn’t get it either but didn’t want to look dumb by asking…thank you for your persistence. 
Not mine, but a classic:
“Daddy, why is the sky blue?”
“So when you’re mowing the lawn, you’ll know where to stop.”
Another one I’ve remembered, and I’ll be sure to use if the occasion arises. Steven King, being responsible for an awful lot of waaaaaay out there fiction, gets more than his share of the question “where do you get your ideas?” He finally started answering this question the same way every time. “Utica.” He says he doesn’t know what it means either, but it shuts 'em up.
My husband’s grandmother apparently did the same thing with a slightly different twist; when asked for X, her response was, “I’ll X you one, right in the arse.” For example, if asked for a dollar, her response would be, “I’ll dollar you one, right in the arse.” I only wish I could have known her better.
On a slightly different note, this thread is a great example of how people mistake being a smart-ass for being funny. Not the same thing.
Yes, and even before we got from “funny” to “smart-ass,” we went from “offbeat” (per the OP’s request) to “funny.” I think he was originally asking for head-scratchers, not knee-slappers. :dubious:
My daughter asked me (in a roundabout way, seeing as how she’s only 7, and hyperalert to the avoidance of salty language) what a smartass was. After thinking about it for a while, I said it was someone who, when asked a question, provides an answer that is absolutely true, but completely and deliberately unhelpful. She was silent for a moment, then said, quietly, hoping not to offend…“But that’s what you do, Daddy.”
I love my daughter.