When you’re setting out to repel pests by using hot peppers, you want the hottest ones your supermarket has to offer. What provides the steamy quality in these fruits is certain heat-producing chemicals, the prime one being capsaicin. How much capsaicin a pepper has—and therefore, how much firepower it delivers—is measured in Scoville Heat Units. (They’re named for scientist William Scoville, who came up with the system back in 1912.) Here’s Dr. Scoville’s scorecard. You might want to keep these numbers in mind when you’re shopping for peppers to use in my Marvelous Mix recipes.