We suspect that people have an everyday theory of mind because they explain and frequently talk about the behaviour of others and themselves in terms of beliefs and desires. Having a theory of mind means that we believe that other people have minds like ours and that we understand the behaviour of these others in terms of the contents of their minds: their knowledge, beliefs and desires. But how can we demonstrate experimentally that people are using their theory of mind to predict the behaviour of others. This problem is particularly acute in the case of animals or young human children when they do not have language. Dennett (1978) discussing Premack & Woodruff's (1978) seminal paper ‘Does the chimpanzee have a theory of mind?’, suggested that the use of false beliefs to explain behaviour would provide convincing evidence. When their belief is true (i.e. corresponds to the actual state of the world) we can explain peoples' behaviour on the basis of the state of the world without needing to know about their beliefs. This ambiguity does not arise when the belief is false. The first experiment to use this approach was published by Wimmer & Perner (1983). They showed that at around 4 years of age a child knows that Maxi will look for his chocolates where Maxi believes them to be, even though the child knows that this belief is false because he has seen Maxi's mother moving the chocolates. In the English-speaking world the task involving Maxi and the chocolates has become the Sally-Anne task (see this issue, Lee et al. 2004).