Chatbots that can answer questions and write things for you are in the news at the moment. These Artificial Intelligence (AI) programs are very good now at writing about all sorts of things from composing songs and stories to answering exam questions. They write very convincingly in a human-like way. However, one of the things about them is that they often get things wrong. Apparently, they make “facts” up or as some have described it “hallucinate”. Why should a computer lie or hallucinate? What is going on? Writing postcards will help us see.