Banking at night from the seat of a raggedy mountain bike, as a 16-year-old living on my own, I taught myself how to game the system. I leaned over my handlebars in the green light of the ATM screen, fed an empty envelope into the hole in the wall, pressed “cash back,” and waited. Every time, to my surprise, the machine ate the empty envelope and regurgitated a $20 bill back at me. With it, I’d buy a large bag of frozen bean-and-cheese burritos. The bank would call later; I’d apologize, say I forgot to put the check in the envelope. The hustle worked only if I did it every now and then, when I needed it, no more than once a month. I never thought to punch in a number higher than 20.