For today's lucky ten thousand, the joke is that

> There are only two hard things in Computer Science: cache invalidation, naming things, off-by-one errors.

I thought there were 3 difficult problems: naming things, cache invalidation, , and off by one errors. concurrency

the concurrency twist got a laugh out of me, I've seen this joke a zillion times but never the concurrency bit

Why do people say that, when the number one hardest problem is making good abstractions?

Because it’s a “famous” (in our circles) quote. You might prefer this one:

> There’s two hard problems in computer science: We only have one joke and it's not funny.

There are at least one more joke:

"There is 10 kinds of people, those who can read binary and those who can't."

Personally I prefer the cache invalidation one.

> "There is 10 kinds of people, those who can read binary and those who can't."

I like the continuation (which requires knowledge of the original): “And those who didn’t expect this joke to be in base 3”.

Names abstract things.

You explained one thing but introduced another needing explanation.

https://xkcd.com/1053/