The high diffusion of smartphones in the users’ pockets allows to sense their movements, thus monitoring the amount of physical activity they do during the day. But, it also gives the possibility to use these devices to persuade people to change their behaviors. In this paper, we present ClimbTheWorld, a serious game which uses a machine learning-based technique to recognize and count stairsteps and aims at persuading people to use stairs instead of elevators or escalators. We perform a fine-grained analysis by exploiting smartphone sensors to recognize single stairsteps. Energy consumption is widely investigated to avoid exhausting smartphone battery. Moreover, we present game appreciation and persuasive power results after a trial experiment with 13 participants. © 2016, Springer-Verlag London