system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do smoke detectors know that there's smoke around? And how do they make it infinitely reusable?
gpt
Inside a smoke detector, there is a small amount of radioactive material between two electrically conductive pieces of metal.  The radioactivity causes the air between the plates to ionize and conduct electricity.  When smoke enters the detector, this ionization is interrupted.  This causes a change in current between the plates, which the detector can sense.

ETA - This is also why the detector is reusable.  There's nothing about this setup that would change after smoke is detected.  The half-life of the radioactive material in the smoke detector is about 400 years, so you don't have to worry about that part either.