Capsaicin

Chillies are hot because they contain an irritant alkaloid, a chemical that acts directly on the pain receptors in the mouth to produce a burning sensation. The body reacts by secreting endorphins — these are natural painkillers that cause a physical ‘high’, thought to account for the addictiveness of eating chillies. Capsaicin is primarily found in the membrane and seeds of the chilli. It is not very soluble in water (water does little to extinguish the heat) but is soluble in oil and alcohol (this is why milk, yoghurt and beer help relieve the heat of curries).