[red_trumpet]

Yeah, finding out if a number is prime or not can be unintuitive. But I doubt Grothendieck would have had trouble with anything divisible by 2 or 5...

[everybodyknows]

My math is rusty enough to find it helpful to explicate that 2*5 is the complete factorization to primes of 10, and our customary arithmetic base for numbers written on the page is likewise 10.