As my grandpa always says, "If they wanted work to be called fun, they wouldn't have called it work," which explains why he retired young.

A number of successful apps like NimbleBit's or Zynga's were designed in a way that work seems like fun. And work is the correct word to describe what a human does with these programs.

These apps are progress simulations. They create the sense of change, of things happening by constantly providing you with attainable goals and rewards for meeting said goals. People like this because people like the sense of doing something to completion.

Of course you aren't really doing anything. You're simply checking in regularly to click a button, receive a reward, click a button, set a new goal, and repeat.