Hello, I just baught my next set of textbooks and started reading about relativity. In one of the books it uses the example of a two clocks who "tick" every time a photon it emitted hits the mirror and returns to the sensor. It demonstrated that, if the box containing this clock is moving, it will tick slower than one that is stationary. I understand this and why, but I dont understand how this is supposed to show time dilation? If I perform the same experiment but with a clock that shoots a tennis ball, while fixing the tennis ball's speed at a constant value, the moving clock, even at speeds far below the speed of light, will still tick slower than the stationary one, yet there would not really be time dilation.