Miranda lists Mom's rules for living in New York City: have your keys out, avoid strangers, and never show your money.
Miranda has her own trick: ask the stranger ("usually a boy") what time it is. Doing so says to the stranger, "I see you as a friend, and I don't have a watch. Therefore, I'm probably not worth mugging."
So far, according to Miranda, it's worked "like gangbusters" (8.3).