Capsaicin from chillies in spicy food binds with sensor receptors in the mouth (tongue, mucous lining, palate … just about anywhere in the mouth). These sensors convey to brain that whatever is in the mouth is ‘too hot’: “Yo, the mouth is on fire! Do something …”. The feeling gradually dies down, but before it does, the brain tells the various parts of body to flush out the offending stuff: nose (sinus fluids), skin (sweat), eyes (tears), mouth (saliva).
Calcium ions detach the capsaicin molecules from the sensors, or desensitize the sensors. Calcium compounds in milk, yoghurt, buttermilk (all milk products) do this quickly, which is why people who know the trick (most people don’t) take a mouthful of cold milk and hold it in the mouth till the feeling dies down.
Chilies contain a substance called capsaicin that makes taste buds sting. When capsaicin pings taste buds, these heat-detecting neural sensors send a message straight to the brain: “fire!” Milk, however, contains casein, a fat-loving compound that binds with spicy capsaicin oil and then washes it away.
Apparently when it comes to beating a mouth-sizzling burn, milk really does the body good.
Source: Quora
Education by: Annie Semanu Dovi and Selasi Kumi-Agbodza