An alarm clock is a clock designed to wake you up at a specific time by making a loud sound. You set it before going to bed, and when the chosen time arrives, it rings, beeps, buzzes, or plays music until you turn it off.

Before alarm clocks, people had to rely on roosters crowing at dawn or pay someone called a “knocker-upper” to tap on their window each morning. The mechanical alarm clock, invented in the 1800s, changed everything. Suddenly, factory workers could arrive on time, students could wake up for school, and people could catch early trains without depending on the sun or another person.

Today, most people use their phones as alarm clocks, but traditional alarm clocks still sit on nightstands everywhere. Some have gentle sounds that gradually get louder, while others jolt you awake with urgent beeping. The trickiest part? Reaching over to hit the snooze button, which gives you a few extra minutes of sleep before the alarm sounds again.

People also use alarm clock as a metaphor for anything that suddenly makes you aware of something important: “Failing that quiz was an alarm clock that I needed to study harder.”