When you eat chilli, the food itself does not get hotter. So why can your mouth feel as if it is burning? The key is not the thermometer reading but the signal received by sensory nerves.
Capsaicin binds to TRPV1 ion channels on sensory nerve endings in the mouth. These channels are also activated by harmful heat at around 43°C or above and by acidic conditions. When a channel opens, positively charged ions enter the cell, generating electrical signals that the brain experiences as burning and pain. The body may also respond by sweating.
People often say that chilli “tricks the brain”, but that is only shorthand. The nerve signal is real; what opened the channel was capsaicin rather than a high temperature. TRPV1 is a little like a doorbell that can be rung by either heat or a chemical. The analogy has limits: the body has other channels involved in temperature and pain, and TRPV1 responses also depend on conditions such as acidity and inflammation.
The research recognised by the 2021 Nobel Prize in Physiology or Medicine began with capsaicin and helped reveal how temperature is converted into nerve signals. Chilli does not raise the temperature; its chemistry recruits part of the pathway used to sense painful heat.
References
Nobel Prize, Press Release: The Nobel Prize in Physiology or Medicine 2021
https://www.nobelprize.org/prizes/medicine/2021/press-release/
NCBI Bookshelf, TRPV1 Receptors and Signal Transduction
https://www.ncbi.nlm.nih.gov/sites/books/NBK5260/
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