Jo and the Bush

I'm physically incapable of living shorter, longer or even the same amount of time. With that out of the way, can you at least give me a hint of where's the joke?

There's an old saying about assumptions that goes "You know what they say about assumptions: You make an chull out of U and ME" Because chull, U, and ME spells out Assume. This saying means that when you assumes something is true without evidence, you're often wrong, and mess everything up for everyone.

I took the first part of the saying: "You know what they say about assumptions: You make---" And then an extreme ending onto it. I set up an expectation for something slightly funny, but switched to gallows humour.

The joke comes from the juxtaposition of the lights amusement of the original phrase with the gallows humor or everyone dying because you messed up.

the emoji serves as punctuation and emphasis of the desired expression and emotion felt by the reader. a blank 'What' face, due to their confusion because of the bait and switch.

Twilight went further, playing off of another old joke. "That's what they say" is an old phrase. some people have made the 'They' into a joke. Who is the 'They'? Why do the 'They' say such things? That joke is simply in taking a metaphorical phrase literally. So twilight reacted literally to my metaphorical joke, extending the humour.

When you replied that you didn't get it, i replied with 'Good. You'll live longer' calling back to my original joke. Because you didn't assume what the joke was, you won't be one of the corpses of people who assume, like Twilight will be, if this joke was actually literal.

TL;DR: The jokes was Bait and switch Gallows Humor.