A chili pepper can make your mouth feel as though the temperature suddenly jumped.
Usually it did not.
The sensation comes largely from a chemical called capsaicin.
Capsaicin activates sensory receptors that normally help warn you about actual heat and physical irritation.
Your brain gets the message and responds accordingly.
Capsaicin activates heat-sensitive receptors
Nerve endings in the mouth and skin contain a receptor often called TRPV1.
This receptor responds to potentially damaging heat and some irritating chemicals.
Capsaicin fits into the system and activates it.
The nerve sends a signal associated with heat and burning.
Your brain does not receive a little footnote saying, “Do not worry, this is only salsa.”
It receives the alarm.
Is spicy food actually burning your mouth?
Not in the ordinary thermal sense.
A room-temperature pepper can produce an intense burning sensation without raising the tissue temperature to the level that caused the sensation.
Very high concentrations of capsaicin can irritate tissue, but the familiar “hot” feeling from food is mostly sensory signaling.
Why do you sweat when you eat spicy food?
Once the brain interprets the signal as heat, it can activate responses normally used during overheating.
You may sweat.
Your face can flush.
Your nose can run.
Your eyes may water.
The body is reacting to a heat message even though the pepper did not turn the inside of your mouth into an oven.
Why does water not help much?
Capsaicin does not mix especially well with water.
A drink of water may briefly move the sensation around without removing much of the capsaicin from the tissues.
That is why water can feel disappointingly useless after a very hot bite.
Why can milk work better?
Fat and milk proteins can help loosen and carry capsaicin away from receptors more effectively than plain water.
The exact relief depends on the food and the person, but dairy products often work better than water for this reason.
Why do people enjoy something that hurts?
The burning sensation can be unpleasant, but spicy food also comes with flavor, aroma, cultural habits, expectation, and sometimes a rush of physiological arousal.
People differ enormously in how much capsaicin they enjoy and tolerate.
Repeated exposure can also make the experience more familiar.
So spicy food is not really setting your mouth on fire.
It is pressing the fire-alarm button.
Some people apparently enjoy dinner with the alarm going off.
A related bit of kitchen science appears in Why does your nose run when you cry?, where a different familiar food has its own chemistry.
And that’s the short explanation.
Related explanations: Why does your nose run when you cry?, Why do we blush?, and Why does soda fizz?