If there's one thing you want people to learn, what would it be and why?

Critical thinking, so everyone could understand that everyone else has their own shit that matters to them.

The world would be a lot nicer to live in if entire groups of our society didn’t feel this incessant need to convert others to their way of thinking, be it political, cultural, or religious.

As long as one person isn’t hurting or subjugating another, IDGAF.

Drive slower, you're not in a rush, it's all in your head, your home's not gonna permanently locked away if you're late, your office won't explode if you're late and if you're already late to work you're already late.

Drive. Slower.

Basic fucking decency would be a start

Empathy

to love themselves so they can love everyone else

Another language. It changes the way you see the world

"Critical Thinking" was already taken, so I'll go with my similar-but-different answer of critical evaluation of sources/information. When I was a kid, we were never to let anyone in when home alone, even to just use the phone. We were told never to believe anyone on the other side of that door that wasn't family or emergency services (and even then to call the neighbors in the latter case). Today, so many of the same people who told me this are fooled by dis- and mis-information online or believe very poor sources. Sometimes, it does fall into the trap of "my sister/brother/pet albatros shared it so it must be true!" of thinking the source is already verifies. We all should get better at this.

Basic computer literacy.

To use the bins/trash cans and stop littering. Especially on beaches, parks, reserves and on the motorway.

Empathy, and I expect the why speaks for itself.

The fact that psychedelic drugs like mushrooms and LSD aren't as dangerous as media and politics make them out to be.

They are actually among the physically safest drugs out there, even when including caffeine and sugar. They can be used in so many ways for self-improvement and treating depressions, anxiety, PTSD and many other conditions.

The book 'How to change your Mind' by Michael Pollan is a wonderful read on the topic.

Maybe it's not just "one thing", but ethics. How to make decisions in a systematic way; how to do it in advance; how to weight morality, practicality, and aesthetics to reach a decision that you'll be satisfied with twenty years later, a decision you could explain and defend to another ethical person before or after the fact.

Critical Thinking skills. Imagine a world where everyone is able to sus out the lies, separate fact from fiction, and not fall into pitfalls of illogical thinking.

How to not be evil. You'd think it's simple, but evidently not.

Situational awareness.

Empathy. People criticizing each other often make the same "mistakes" that are nothing but normal human behaviour. Once you understand that we are all pretty much equal, you start realizing that most bad things are sistematic. There are few bad people, most people are quite nice and forced (or taught) to behave badly

I wish we could learn from our mistakes.

Gender alone doesn't need to block you from doing things.

Probabilities and basic stats. People do not think in "what are the chances" but in black and white. I think one reason is we don't teach probabilities in American schools. It drastically impacts a citizens ability to understand the news, and especially science.

How to clone and build a git project

Responsibility, way too many people do things without any thinking and making everything into shitshow because of not thinking

How to properly go through a four-way stop.

My gut answer was math.

Yeah it’s not as important as decency. But fucking hell people, it’s not that scary and it teaches you to think in ways a lot of people could use to think

Class consciousness.

So then we stop fighting each other for peanuts and look at who gets to benefit from our generalised political apathy.

Class struggle

Me: Composting

Interesting choice.

Recently local people were forced to separate bio waste from regular waste or compost bio waste themselves.

There were so many smelly failures around town for a long time and many gave up. Just small mistakes.

That they need to understand others in order for people to understand them. That the "tragic prince" is just a fallacy and I would really really want other people around me to appreciate art-forms more. Most of the time they find a movie good and just list the content as the reason for it's goodness, not paying attention to any of the craft and it baffles me that more people are not attached to or interested in how art-forms do the things they do.

Digital privacy matters as much as physical privacy & we need to keep the governments & corporations out since they can constantly surveil. Method for doing so need to legal, cheap, & accessible. If decentralization is a requirement, you system that requires Amazon S3 buckets & a beefy VPS are not sufficient when these sorts of things rarely have a technical reason why they couldn’t be democraticized to run from an apartment (why some ISPs don’t let you have an IP (v6 or not) or symetric connections as bits are bits is a different matter).

how to communicate

How to properly zipper merge on a highway.

Sense of the, so called, common variety, ironic wording considering how uncommon it is. But I imagine that educating people would help this, both in school and in the community. One seemingly obvious thing would be how people litter in green spaces, was that wrapper going to kill you when you get home?

you want [...] what would it

The first thing that pops into my head is an idea about writing. You're mixing your tenses.