As die-hard Chileheads know, the burning sensation that makes chiles so appealing to culinary thrill-seekers comes from capsaicin, or, more accurately, a collection of compounds called capsaicinoids. These develop in the placenta, or cross-ribs, of the fruit, which is why that part of the chile is the hottest. A single dominate gene transmits capsaicinoids. Bell peppers are Capicum annuums just like Jalapenos, Serranos, Thai Hots, and most other hot peppers you might find in a grocery store. But Bells taste bland instead of pungent because they lack that gene.

You can't smell most capsaicinoids, and apart from their bite, most have no taste. But they're so potent that some tasters have reported they could detect capsaicinoids in a solution containing only one part per thirty million.

Early in this century, a pharmacist named Wilbur Scoville devised a way of scoring capsaicinoid content: Scoville Units. Most peppers fall into a range from zero to 300,000 Scoville Units. Bell peppers rate zero because they contain no capsaicionoids. Mild peppers like New Mexico 6-4's earn 500 to 1,000 units. Jalapenos measure 2,500 to 5,000. Both Tabasco peppers and Cayennes rank 30,000 to 50,000. Wild Chiltepins receive 50,000 to 100,000 units. At the top end, peppers like the Caribbean's Scotch Bonnet and the Yucatan's Habanero earn up to 300,000.

These days many chile writers use a new system, the Official Chile Heat Scale, with a rating of zero to ten. Bell peppers still fall to the bottom, with zero; Scotch Bonnets and Habaneros still fly to the top, with ten. In between come Jalapenos at five, Serranos at six, Cayennes and Tabascos at eight, and wild Chiltepins and Thai Hots at nine.

Both ratings sound scientific enough, but both rely on nothing more precise than the perceptions of the tasters.

Learn more about chiles in our Chile Glossary.