You've probably encountered silent light switches before. Unlike regular light switches, which make a distinctive click, silent light switches operate smoothly without a sound.
This video takes apart a silent light switch to reveal the secret — it contains a drop of liquid mercury in a sealed capsule. When the switch is flipped, the mercury rolls over two contacts to complete the circuit. These kinds of switches are no longer sold because mercury is toxic, and if it leaks from the sealed component, it will contaminate the building.