Drinking beer. Yes, seriously. I especially remember one Friday afternoon in the late 90s when I was still a trainee at a major Swedish telecom company. I had been working on a test application which gave visual output in the shape of dots teeming around on the screen, each one of which represented a network node. Then my colleagues and I had an afterwork at a nearby bar. After a few pints, when the others went home, I returned to the office and, in an inspired mood, made a few modifications to the test app so that you from each client could control one of the dots with the keyboard, basically turning the app into a multiplayer game. Over time I improved it further with some sprites and the possibility to shoot at each other. We had great fun while performing tests :D