Dr Zhihao Yan asks a class of primary children where they would go if they had one trip in a time machine. The answers map what each child has been allowed to be curious about. One girl goes to Tang-dynasty Dunhuang to find out how the library cave was sealed; a boy to the Cretaceous to see a triceratops, and to the Cambrian, and to bring something back as a pet; another to Stonehenge, because of the rumour about aliens. A boy whose parents are migrant workers, and whom the project funds, chooses the extinction of the dinosaurs and the evolution of human beings. One girl chooses the first day of a school term, because she once failed to hand in her holiday homework. He sets this beside a boy in 2018 who said he wanted to become a tiger — and, after half a minute of silence, that it was because tigers eat people, and people set homework.

In 1897 the New York Sun told eight-year-old Virginia O'Hanlon that yes, Father Christmas exists. A century later a New Jersey teacher who told first-graders the opposite reportedly lost her job. Dr Zhihao Yan uses the contrast to open a question he has actually put to his own primary classes: does Father Christmas exist, and how would you argue it? About one child in ten says yes, and their reasons turn out to reconstruct Pascal's wager almost exactly — believing costs nothing and might pay. The majority say no, and one group builds a proof by contradiction listing the seven superpowers he would need. What the exercise shows, he argues, is that children are not confused by make-believe: they move in and out of it deliberately, and asked to defend a position they step outside the story and reason about it in the third person.