When making a joke, it's important that the premise remain within the bounds of accepted reality. Stretching those boundaries is okay, but it needs to be done gradually, and the result has to be a NEW accepted reality. Otherwise it's just not funny.*
Let me give you two good examples of well-done humor that fits WITHIN an accepted reality: P.G. Wodehouse's Bertie Wooster thinks he's clever and intelligent. But his bumbling ends up making every situation he's in worse. THEN he goes to his genius butler, Jeeves, for help. Often enough, the simplest solution is the one Jeeves thought of. And he's typically performed the required tasks to solve Bertie's problems. The jokes always play on Bertie's incompetence and Jeeves's genius. Not much of a stretch in reality, right?
Example two: The entire Hitchhiker's Guide to the Galaxy series. A two-headed intergalactic president is funny. Not because it's plausible in a real-world sense, but because when we pick up a sci-fi book, we ACCEPT the possible existence of two-headed aliens. And when we accept that reality, it's not much of a stretch to accept a spaceship with an infinite improbability drive. And when we accept THAT reality, a bowl of petunias that's capable of thinking, "Not again?" is just plain laugh out loud hilarious.
Now for the flip-side: I have a broad sense of humor, and generally Whose Line is it Anyway is funny to me. But every time the actors say something preposterous like, "I'll use my outie bellybutton to scale the wall," I cringe. It's not funny. At all. That's not my opinion, that's a completely objective, indisputable fact. Okay, maybe I'm overreaching, but I just think it's stupid. Not funny. But my point is, the reality is beyond acceptable to me.
Finally, I want to give you some homework. Go find two movies and/or TV shows. One must be something you find uproariously hysterical. Examine it for the jokes, the reality in which they take place, and then see if you can figure out WHY you think it's funny. THEN go find a movie that EVERYONE ELSE tells you is funny but you think is the stupidest, most boring film of ALL TIME. Examine that one, too. What jokes don't you like? Why? Is there some way you might laugh at those jokes?
If that's too much, just give two examples from memory in the comments. And I'll see you on Friday!
*As with any rule, there are exceptions, but this discussion isn't about those exceptions. Just like every other set of rules, learn to master them before figuring out how to break them.