For those who are unfamiliar, capsaicin is one of the most interesting molecules out there. This is the chemical that makes chili peppers spicy; it is also the active ingredient in pepper spray. Pure capsaicin is rated at about 15 million Scoville heat units: a scale where Tabasco sauce is scored at about 2,500.
Capsaicin is quite an amazing adaptation, actually. Plants developed it to deter animals from eating them. Eventually, animals realized that capsaicin was painful to them, but not actually harmful. By contrast, it is very harmful to some of the species of bacteria that spoil food. As such, spicy foods emerged in hot climates as a defence against nasty prokaryotes. A direct descendent of that realization are the veggie vindaloos I enjoy so much.
Spices have been mentioned here before.