How does a smoke detector sense smoke?

A smoke detector notices smoke by watching a tiny electrical signal inside itself. In one common kind, a sealed bit of americium helps make charged air particles. Clean air lets the signal flow normally. When smoke drifts in, it steals those charges away, the signal changes, and the alarm sounds.

The lovely detail is how tiny the change can be. The detector is not smelling smoke like a nose. It is noticing that floating soot has blocked an electrical pattern, like dust interrupting a very quiet conversation between two metal surfaces.