capsaicin, it's 'hot' because it contains something called substance P was identified by this guy way back in teh day, before you could identify chemicals as easily as we do now a day. He actually bottled a bunch of it in powder form, and it was promptly lost for about 50 years, until it was found in some of his affects his children went through one day. It triggers a release of a polypeptide acting at nerve synapses, and because of it's nature once the release of it starts, it keeps going until there's none left in the nerve. This is why drinking water and other liquids doesn't dull the burn when you eat hot foods, only time does.
I see someone posted a capsaicin link, good read