Put a thermometer on a slice of habanero and it reads room temperature. There is no heat in it at all. Yet your mouth insists it is on fire, you sweat, your face flushes and your eyes water. Your body is responding to a burn that isn't happening.
The culprit is one molecule, capsaicin, and one tiny gate in the nerve endings of your tongue called TRPV1. Its normal job is to warn you about real heat. It opens when tissue gets hotter than about 43 °C, the point where scalding starts, and lets charged particles rush into the nerve. The nerve fires, and your brain reads the signal as "hot, pull away".
Capsaicin fits into a pocket on that same gate and pushes it open too. The nerve has only one way to report "my gate is open", so the message that arrives in your brain is the same one real heat sends. You aren't tasting spice. You're hearing a false fire alarm.
Below is a patch of nerve membrane with TRPV1 gates in it. Drag on the thermometer (or the picture) to change the temperature of your mouth, add some chilli, then try to put the fire out.
Notice what happens with the hot tea after a chilli. Capsaicin doesn't just open the gate on its own; it also makes the gate easier to open with heat. Lab studies show the two effects add up: with capsaicin around, TRPV1 responds more strongly to heat that would normally be tolerable. That's why hot soup after a spicy bite feels so much worse than the same soup on its own.
In 1912 an American pharmacist, Wilbur Scoville, wanted a way to rate peppers. He soaked dried pepper in alcohol to pull out the capsaicin, then diluted that extract in sugar water, step by step, and gave it to a panel of five tasters. The rating is how far you have to dilute before at least three of the five can no longer feel any heat. A pepper rated 5,000 Scoville heat units needed about 5,000 parts of sugar water to hide it.
Human tongues are unreliable instruments, and tasters get numb as a session goes on, so results from different labs could differ by as much as 50%. Since the 1980s labs have mostly measured capsaicin chemically instead, using liquid chromatography, and converted the answer back to Scoville units by multiplying parts per million by about 15. The scale survived; the tasting panel mostly didn't.
Capsaicin barely dissolves in water: only a few milligrams would dissolve in a whole glass. So a gulp of water mostly spreads the oily molecule around your mouth while leaving it stuck to the gates. The brief relief you feel from cold water is only the temperature drop, and it fades as your mouth warms back up.
Milk does better. Its proteins, called caseins, help pull capsaicin away into the drink so you can swallow it, which is why milk and yoghurt sit beside so many spicy dishes. Capsaicin also dissolves in alcohol, which is how Scoville extracted it, but a weak beer is mostly water and won't do much.
Keep eating chillies and the alarm also gets quieter. With prolonged exposure TRPV1 becomes desensitised: the nerve endings stop responding as strongly. That's part of why regular chilli eaters tolerate far hotter food, and it's why capsaicin creams are used, oddly, as painkillers.
Why would a plant make a molecule that fakes a burn? Look at who gets fooled. The TRPV1 gate in birds doesn't respond to capsaicin, so birds eat chillies happily. Mammals, whose molars grind seeds up, get the fire alarm and leave the fruit alone. Birds swallow the seeds whole and drop them far away, ready to grow. The heat is a filter that picks the best seed carrier.
The gate itself was only identified in 1997, in David Julius's lab, by searching for the gene that made cells react to capsaicin. That discovery earned him a share of the 2021 Nobel Prize in Physiology or Medicine, with Ardem Patapoutian, for finding the body's sensors for temperature and touch. Menthol plays the same trick in reverse on a cold sensor, which is why mint feels cool at room temperature.
So next time a curry makes you sweat, remember: nothing is burning. A plant has hacked your heat alarm, and your whole body is politely responding to a fire that isn't there.