A smoke detector is a device that senses smoke, typically as an indicator of fire. Smoke detectors are typically housed in a disk-shaped plastic enclosure, about six inches in diameter and one-and-a-half inches deep, with an outer cover that can be removed to replace the batteries or to test the unit. When smoke is detected, the detector sounds an alarm that is loud enough to alert people in the area, and often has a blinking light to indicate it is detecting smoke.