I understand time dilation exists according to relativity theory. But what I'm not able to understand is this statement, "clock is ticking faster/slower on a moving object compared to observer". How is that possible to measure it? or how does the clock (atomic/quartz) it experience the time difference? Because, lets say for atomic clock, the International System of Units (SI) has defined the second as the duration of 9192631770 cycles of radiation corresponding to the transition between two energy levels of the caesium-133 atom. Does number of cycles change when the atomic clock moves in order to observe the difference in time? Or more simply (lets say quartz clock) I'd imagine, frequency of quartz crystal (the physical characteristics) would be same no matter how fast it moves? Then how can we measure the time difference accurately. I'm sure I'm missing something and I'd really appreciate any insight to that.