Tick tock, tick tock—that's the sound we think of when we think of clocks, even though the vast majority of modern timekeepers make hardly any sound at all. Not so long ago, virtually every clock and watch made a tick-tock noise because it was completely mechanical rather than electrical or electronic. Instead of your clock being powered by a battery, you wound it up with a key and there was a long swinging rod inside, called a pendulum, that made sure the whirring gears kept good time. So how did these old-fashioned pendulum clocks actually work? Let's take a closer look!