For decades, developmental psychologists have been fascinated with the question of how children develop theory of mind – in other words, how we come to understand that other people can have different types of thoughts, beliefs and knowledge to ourselves. A key milestone in this journey involves developing a notion of false belief; sometimes, the things that people believe about the world are very different from the reality of the situation, and this will have important consequences for how people act. But how do you measure something so seemingly esoteric?