A smoke detector is a small device, usually mounted on a ceiling, that sounds a loud alarm when it senses smoke in the air. Inside the device, special sensors constantly monitor the air. When smoke particles enter the detector, it triggers a piercing beep or siren to warn everyone in the building that there might be a fire.

Smoke detectors save thousands of lives every year by giving people precious extra minutes to escape a burning building. Because smoke often spreads faster than flames, and because fires at night can fill a house with smoke before anyone wakes up, these devices act like vigilant guards that never sleep. Most smoke detectors run on batteries or connect to a building's electrical system.

You've probably heard a smoke detector's annoying chirping sound when its battery runs low. That's the device reminding you to replace the battery. You might have also accidentally set one off while cooking, when steam or smoke from the stove drifts up to the ceiling. Modern building codes require smoke detectors in homes and schools because they're such an effective and inexpensive way to protect people from one of the most dangerous household emergencies.