system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how can my smoke detector not be set off by my cigarette or fireplace smoke, yet goes off the moment my toast starts burning.

Not sure this has been asked yet. If it has link me to post? Reddits search function is weird...
gpt
Smoke detectors work by having a radioactive substance and a radiation detector inside.  The detector keeps getting the emissions from the substance, and this keeps a circuit open.

Whenever enough particles that can block the radiation get between the emitter and the sensor, the circuit closes and the alarm goes off.

Fireplaces and cigarettes actually burn pretty cleanly, meaning that the smoke they create is made up of very fine particulate matter that the radiation can get through, unless you've got enough present to be difficult to see through.

Burned toast smoke, on the other hand, creates larger airborne creosote-like objects that can easily block the radiation.  This is due to the oxidation reaction being much less efficient, resulting in larger particulate matter being released into the air as smoke.