QuoteEverybody can read C source code.
Everybody knows the rules of popular classical board/card games.
Everybody is fit with high-school mathematics, in particular, high-school level calculus.
Everybody knows some mathematics beyond the high-school level, whether from formal education or by self-teaching. Hard to define how far I assume this; let's say, complex numbers, some graph theory, linear algebra, elementary set theory, elementary topology.
These ones aren't true for me, and not for most people I see in at least some of the communities I frequent. It does feel like it's true for most people I hang out with in the puzzler community, though.