Why chilli feels hot when it isn’t

The aha, in writing

1. Your mouth is wired with pain-sensing nerves, and on them sits a tiny sensor called TRPV1, the lock on a heat alarm.
2. Its normal job is to open for painful heat, above about 43 degrees Celsius, and ring the alarm.
3. Capsaicin, the molecule that makes chilli hot, switches the same sensor on, so your brain gets the same signal and reports heat.