my first mentee asked why I check for null before the loop and I blanked

Got bumped to senior in March and two weeks later got handed a new grad to ramp up. First week she asked me why I always check for null before the loop instead of after, and I just... didn't have an answer. I'd been doing it for six years without ever needing to explain it. Ended up spending a whole Friday afternoon writing down "rules" I follow that I'd never once written down, and half of them turned out to be habits, not actual reasons. Turns out explaining code teaches you more about your own blind spots than reviewing it ever did. Still not sure I'm doing the mentoring part right.