Metronome
A traditional metronome works by clockwork. It has a pendulum which makes a loud tick as it goes to and fro, like the pendulum of a grandfather clock. It has to be wound up after a few minutes of ticking. There is a weight on the pendulum which can be slid up or down and set to a particular speed. There are numbers on the metronome to which you can set it. These numbers mean the number of ticks per minutes. A speed of 60 will make it tick 60 times a minute (i.e. every second). The numbers usually go from 40 (slow) to 208 (very fast). Nowadays many people use electronic metronomes. These are very accurate, do not need to be wound and often have other helpful things like an A to tune to (see musical tuning). Some of them are hardly bigger than a credit card.