Acknowledgements

Zena, Amir, Maha, Emma, Gerrit, Roel, Ida, Saskia, Maarten, Trudi, Hans and Titus: thanks for playing with me.

Shona, Hans, Petra, Ian, Richard, Marja, Saleh, Greg, Suzanne and Maarten: thanks for sparring and for testing things.

Mike: sorry about your shoe (page 33).

Children complain that the ketchup is touching their peas; they never complain that life is pointless. It occurs to me: life never had any meaning, because it’s not a maths puzzle that can be solved. The secret of life is to play. Andrew Clover

If it exists, play with it. Michael Rosen