The woman is a university lecturer who is known for her unorthodox approach to teaching. In preparation for a forthcoming module she has devised a three-part experiment. Each morning, at about 9AM, the man she has chosen to help with the experiment leaves the Arts and Law Building where she works. She doesn’t know what he does or where he is going – whether he has started work at 8 and is nipping out for a coffee, whether he is going home after a night shift, whether he doesn’t actually work in the Arts and Law Building but is just passing through – but this is irrelevant; what matters is that she arrives at the building as he leaves.

On the day she intends to start the experiment, the woman checks her equipment, which consists of a tiny camera attached to her coat. At about 9AM, she arrives at the Arts and Law Building to discover her timing is perfect. As she enters the building from the campus, the man goes to leave the building’s foyer. The woman and the man catch each other’s eye. The man holds the door to the foyer open for her and the woman holds the door to the campus open for him. Between the external door and the door to the foyer there is a space. The space can be crossed in three seconds or a hurried second and a half, which is not enough time for the woman and the man to simultaneously let go and reach each other’s door before it swings ...