I always liked the ideal photon clock in two different frames of reference as an example of relativistic time dilation.
Photon bouncing between two ideally perfect mirrors if the observer and clock are not moving realtive to one another.
As observed when moving past observer at a high proportion of the speed of light.
Since experimental observation has shown without exception that the velocity of light is the same for ALL observers, regardless of relative motion of observer and light frames of reference (THE fundamental assumption of special relativity physics), the clock in relative motion to the observer will appear to tick-tock
slower (in the frame of the observer) than when clock and observer are stationary relative to one another as in the top animation.
This isn't a trick. The underlying principle applies to all time measurment apparatus since any and all types of clocks could, at least in theory, be synchronized to such an ideal photon clock.
[ok - so I'm bored}