The insidious and derisive claim that “if you can't explain it to a five-year-old, you don't understand it,” is adopted by people who claim to be parroting Einstein (it must be true because everything Einstein said was true, right?). But this claim is false on so many levels that it's difficult to know where to start. Well, firstly, let's start with attribution: Einstein didn’t say this! In fact, Einstein implicitly conveyed the opposite when he stated that he couldn’t enter the 1920s competition to explain relativity in 5,000 words to a general audience. Note, that that’s a general audience—not even a five-year-old, but rather an educated, say, 18-year-old. Further note that you get 5,000 words. Now, you try saying anything uninterrupted for 50 words to a five-year-old, let alone 500, let alone 5,000! Secondly, even Feynman said to a reporter, who was asking him to explain QED: “if I could explain it to you, it wouldn’t be worth the Nobel prize…” Thirdly, what someone calls “simple” is based on one’s own familiarity with the terms, and not the inherent simplicity of the concepts. Indeed, something as “simple” as “logarithm” requires you to know what multiplication is, which itself required months of drills and hammering home in elementary school, which itself requires the concept of addition, and that, too, required months of drills. It’s only after this process of usage and boot camp that you think it’s a “simple” concept. But there’s nothing congenitally “simple” about it! Fourthly, it’s harrowingly often the case that understanding a subject deeply and being able to explain it are only mildly correlated phenomena. For instance, in my alma mater of the University of Toronto, it’s (in)famous for being a research university first and foremost. This means they hire based on a professor’s knowledge of the field, and not on their ability to teach. Being a great explainer and a great understander are different skillsets. Fifthly, why the arbitrary cut off at five years old? Why not fifteen years old? Why not twenty-five? Why not two? (Next up: explaining second-countable Harsdorf spaces to cellular division.) 1/6