I first met Duo the owl before a move to The Netherlands was even a glimmer in my eye, opening duolingo.com on a lark one day at work and discovering how devilishly simple they’d made it to get started. After two or three clicks, I was learning the basics of Dutch: vrouw = woman, jongen = boy. Weeks later, I surprised Kiki on one of her visits to Texas by showing off my fancy new Dutch skills: “De vrouw heeft een appel.” (The woman has an apple.)

It only took a couple of months before Duolingo’s various casino-style “gamification” tricks got me on a daily streak that, as of this writing, stands at 2,500 days. Like most addictions, I never expected it to last this long. Unlike most addictions, it’s adding rather than removing brain cells.