Capsaicin, the compound responsible for the heat in chili peppers, does not burn you. This is worth stating clearly because the sensation it produces is indistinguishable from an actual burn, and the human brain is not set up to tell the difference in real time.

What capsaicin does is bind to a receptor called TRPV1 — Transient Receptor Potential Vanilloid 1 — which is the same receptor your nervous system uses to detect genuinely dangerous heat, specifically temperatures above around 43°C. When capsaicin arrives, TRPV1 cannot tell that no actual heat is present. It fires exactly the same signal it would fire if your mouth were being held against a hot pan. Your brain receives that signal and responds accordingly: sweating, increased heart rate, a sensation of burning.

No tissue is destroyed. No cells are damaged. The 'fire' is entirely in the signal, not in the substance. This is why milk helps — casein, a protein in dairy, binds to capsaicin molecules and washes them off the receptors. Cold water doesn't help because the problem is not temperature; it is chemistry.

TRPV1 receptors are not limited to the mouth. They line the entire gastrointestinal tract, from oesophagus to rectum. This is why eating spicy food is a full-body digestive event. It is also why whatever capsaicin your body doesn't absorb during digestion — and it doesn't absorb all of it — produces the same signal on the way out as it did on the way in.