This comes to us from a book about to be published in late September: The Secret History of Food by Matt Siegel. We can start to taste something sugary when it's 5,000 parts per million...we can sense salt when it's 2,000 parts for million. We can sense capsaicin when it's one part per million. Capsaicin is a chemical compound that makes chili peppers hot. The chili pepper is not a vegetable, it's actually a fruit, a berry. The capsaicin in it is a defense mechanism so predators don't eat it. Birds are immune to capsaicin because they eat but don't digest the seeds. The seeds pass through their bodies and are scattered everywhere. Humans crush the seeds with our teeth so we must be punished. Our nose runs to protect our nasal passages and we cough and maybe sneeze to do that, too, eyes water to protect our corneas, excess saliva gushes to clean our mouth...and this goes for other creatures tempted to eat a chili pepper. Makers of wallpaper have used capsaicin in their glue to keep rodents from chewing on it and some car makers have started using capsaicin in their tape wrapped around electrical wires to keep rodents away. We may have started using chili peppers to preserve food. The crushed seeds kill bacteria and cover the taste and smell of food that isn't the freshest. So you'll find spicier foods in hotter climates where preserving food can be a challenge. And maybe it's our sense of adventure: we have to ride that roller coaster.