Drew Houston, the CEO and co-founder of Dropbox, returned to MIT to deliver a commencement address built around a 'cheat sheet' with three items: a tennis ball, a circle, and the number 30,000. The tennis ball represented the thing that pulls you — not just work you love, but a problem you're obsessed with solving. Houston drew a crucial distinction between loving your work and being obsessed with a problem. He had started an SAT prep company and worked hard at it, but eventually 'snapped' because it wasn't his tennis ball. Meanwhile, he built a poker bot as a distraction — and became possessed by it, thinking about it in the shower and in the middle of the night. 'It was like a switch went on — suddenly I was a machine.' The dog-chasing-a-tennis-ball metaphor captured the quality of genuine obsession: 'their eyes go a little crazy, the leash snaps and they go bounding off, plowing through whatever gets in the way.' Houston contrasted this with friends who 'work hard and get paid well in their jobs, but they complain as if they were shackled to a desk.' The difference wasn't talent or discipline — it was finding the right problem.