Let A B C be three travellers going along a straight road, each at constant speed. Let the following three events happen in succession, the passing of C by A at O (Fig. 1), the passing of B by A at V, and the passing of B by C at W. At the meeting of A and C let A set his watch by C’s, and at the meeting of A and B let B set his watch by A’s. Our question is whether the watches of B and C will agree when they meet at W.