Brains were thought to measure time by using some kind of internal clock that generates events at a relatively regular rate. To test whether external stimuli might also play a role in our ability to process time, Misha Ahrens and Maneesh Sahani at University College London showed 20 subjects a video of either a randomly changing stimulus—statistically modelled on the way that things naturally change randomly in the world around us—or a static image, for a set period of time. When asked to judge how much time had passed, the volunteers who had been shown the moving stimulus were significantly more accurate.