A Heisenberg's dog is the central unit in a quantum computer that factors prime numbers. We report the outcome of a proof-of-principle Gedankenexperiment in which the number 3 was factored and good agreement was found both with an analytical theoretical prediction and with the results of a quantum Las Vegas calculation. Other future applications of Heisenberg's dog are hinted at. The anonymous reports of the referees and our reply to them are reproduced in appendix A and appendix B respectively.