Quantum Mechanics
When you try to understand how very small things work, you realize that you can't use the same kind of ideas which you used to explain how bigger things like cars and balls work. So one of the things you realize is that very small things care about how you look at them. Suppose you have a room with two doors. With big things, if you opened one door and saw a red ball inside and then you opened the other door, you would also see a red ball. But with small things, it could happen that you open one door, see a red ball, open the other door see a blue ball and then open the first door again and now see a blue ball! Also, two very small things that are far away can know much more about each other than two big things that are far away. This is what makes small things much weirder than big things, but also can be used to make better computers and phones. But big things are always made of small things; so why do big things also not work like small things? Well, this is one of the most important questions that people who think about small things are thinking about, but the answer seems to be that when you put a lot of small things together the weird things always seem to kill each other off.
xkcd's Up-Goer Five comic gave technical specifications for the Saturn V rocket using only the 1,000 most common words in the English language.
This seemed to me and Briénne to be a really fun exercise, both for tabooing one's words and for communicating difficult concepts to laypeople. So why not make a game out of it? Pick any tough, important, or interesting argument or idea, and use this text editor to try to describe what you have in mind with extremely common words only.
This is challenging, so if you almost succeed and want to share your results, you can mark words where you had to cheat in *italics*. Bonus points if your explanation is actually useful for gaining a deeper understanding of the idea, or for teaching it, in the spirit of Gödel's Second Incompleteness Theorem Explained in Words of One Syllable.
As an example, here's my attempt to capture the five theses using only top-thousand words:
If you make a really strong computer and it is not very nice, you will not go to space today.
Other ideas to start with: agent, akrasia, Bayes' theorem, Bayesianism, CFAR, cognitive bias, consequentialism, deontology, effective altruism, Everett-style ('Many Worlds') interpretations of quantum mechanics, entropy, evolution, the Great Reductionist Thesis, halting problem, humanism, law of nature, LessWrong, logic, mathematics, the measurement problem, MIRI, Newcomb's problem, Newton's laws of motion, optimization, Pascal's wager, philosophy, preference, proof, rationality, religion, science, Shannon information, signaling, the simulation argument, singularity, sociopathy, the supernatural, superposition, time, timeless decision theory, transfinite numbers, Turing machine, utilitarianism, validity and soundness, virtue ethics, VNM-utility