In this paper, I provide a general introduction to the trolley problem. I describe its birth as a philosophical thought experiment, then its successful career in moral psychology. I explain the different reasons behind its popularity and success but argue that, despite its popularity and widespread utilization in psychological research, few researchers have actually tried to directly solve it and that we are still ignorant of the real factors guiding our responses to trolley cases. Against the idea that trolley problems were just a fad, I insist on the importance of seriously studying them and put forward a new hypothesis about the psychological mechanisms underlying our responses to trolley cases.