Imagine I were to make a light clock by placing two mirrors facing each other, with a photon bouncing between them vertically, such that one complete round trip for the photon takes exactly one second. I understand that if this contraption were moving horizontally at significant speed, an observer with a stationary frame of reference would perceive my clock 'ticking' slower than once per second. This seems to make perfect sense to me because the photon (according to the 'stationary' observer) must travel diagonally in order to hit the opposite mirror. Since this leads to a path which is longer than the straight-line distance that the photon travels according to an observer moving with the clock, I can see that time must change given that speed is constant and distance is increased. So far so good. However where it all falls down for me is when you attach a 'regular' mechanical wristwatch (for example) to the light clock. Special relativity requires that according to an inertial observer, the light clock and the mechanical clock cannot fall out of sync. So it follows that to the 'stationary' observer, both clocks run slower. What I can't get my head around is *why* the mechanical clock slows. With the light clock the distance travelled by the photon appears to change, but there is no travelling photon in the mechanical clock. I'm sorry if this is a stupid question, but it really has me stumped. I will be extremely grateful if someone can explain this to me!