From what I can understand, “Flow” is a theory about how to identify something as fun… sort of. Basically you need a balance of challenge and abililty to cope with challenge to have fun. Read about if for yourself here.

That’s all a bit of a dull read, and I wouldn’t have posted it except that these guys made a highly addictive flash game to demonstrate the concept. I’ve been playing it for about 20 minutes and am starting to understand what’s going on. Make sure you have sound. Play it here.