In formal interviews it is important that interviewees indicate when they do not know the answer, rather than speculate. In this study we investigated whether question format affected the tendency to speculate. One hundred and twenty-eight 5- to 9-year-olds, and 23 adults, were told two short stories, and were then asked questions about the stories. Half of the questions were answerable based on the information provided; the other half were not answerable. Within these categories, half of the questions were closed questions (i.e. only required a yes/no response), and half were wh-questions (i.e. requested particular details to be provided). All participants performed at ceiling with the answerable questions. With the unanswerable questions, there was an effect of format. The majority of children and adults correctly indicated that they did not know the answer when asked unanswerable wh-questions. However, the majority of children, and just over one-fifth of adults, provided a response (i.e. ‘yes’ or ‘no’) to the closed unanswerable questions. The implications for interviews, particularly within a forensic context, are discussed. Copyright © 2001 John Wiley & Sons, Ltd.