It took about 50 hours of interaction to move from acquaintance to casual friend, about 90 hours to move from casual friend to friend, and more than 200 hours to qualify as a best friend. Those who never got beyond being acquaintances usually had spent no more than 30 hours together — the equivalent perhaps of being in the same class a few hours a week, but not seeing each other outside of the classroom. On the other hand, time together didn’t automatically make two people friends. Some adults reported spending hundreds of hours with colleagues, but still called those people acquaintances. Basically, they just didn’t like them very much. Or they had no relationship outside of work.