system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5, Why do smoke detectors require batteries, when they are plugged into the wall?
gpt
Smoke detectors don't get power from the wall.  In cases of fire or other situations where you would really want your smoke detector to work, power outages are common.  The fire could spread in between your fuse box and the smoke detector and cut off power to it, so it uses a battery.