We have investigated the use of shared mobile displays to create a new type of computer games for mobile devices - Collaborative Games, which require players to physically coordinate their activities to succeed. Collaborative Games are played on mobile devices connected in wireless networks where users can start, join and leave games ad hoc. In a user study, one of these mobile games was made available in a café frequented by high school students for a period of two weeks. During the test period we noted several new forms of interaction emerging, such as players running away with their displays to avoid other players from accessing them. We also found interesting verbal exchanges, such as the use of "me" to refer to both the user's handheld display and her on-screen representation. We believe that these new ways of interaction is a result of using the shared display in a new domain.