Now, there's no fountain for a while. Because you didn't write it down in an FAQ, and the people that figured Everybody Knows about it are now outnumbered by the people who don't.
This is an example from game fiction, but the same happens for community norms. Hell look what happens here whenever Twitter does something stupid. Everybody Knows we don't do screenshot dunking for clout chasing here, until a few thousand people pile in and suddenly we don't.
If Everybody Knows, then WRITE IT DOWN!