A clock does not measure time the way a ruler measures length. There is nothing to hold up against "a second". What a clock really does is count something that repeats at a steady pace, and show you the count.

That "something that repeats" can be a swinging weight, a spinning wheel, a vibrating crystal or a vibrating atom. Whatever it is, every clock, ancient or modern, is built from the same four parts:

• An oscillator: the thing that repeats. It makes the same motion over and over, and it is the part that sets the pace.
• A counter: something that adds up the repeats (gears with teeth, or electronic circuits).
• A power source: a falling weight, a coiled spring or a battery, because every real oscillator loses a little energy each time it swings and would stop without a top-up.
• A display: hands on a dial, or digits on a screen, turning the count into hours, minutes and seconds.