How can we begin to empirically analyse computer games as dynamic systems that seek to motivate and facilitate meaningful and pleasurable player activity? This paper suggests that the concept of ‘challenge’ offers a promising starting point for such inquiries. First the notion of challenge will be briefly introduced. Then the The Sims and The Sims 2 will be analysed and compared with regards to their challenges in order to highlight some of their distinct characteristics as games and to identify some of their differences. These games are interesting in terms of challenge because they, according to a narrow definition of games, are borderline games due to their lack of clear overall goals and a winning condition. Still, challenge seems to offer a fruitful frame of understanding, thus showing promise as a foundation for an extended definition of games.