In this problem, we consider a light clock$--$a clock that ticks every time light makes a round trip between two mirrors separated by a distance L$.$ The key point is that we can deduce the rate at which this clock ticks, when it moves relative to us as well as when it is at rest, directly from the postulates of special relativity. The postulates of special relativity are as follows:$1.$ The laws of physics are the same in any coordinate system that moves at constant velocity $($i$.$e$.,$ any inertial reference frame$)\mathrm{.}2.$ The speed of light is c when measured with respect to any coordinate system moving at a constant velocity.