Why are chillies so spicy?!

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Capsicum is a genus of plants from the nightshade family native to Mexico, and now cultivated worldwide. Some of the members of Capsicum are used as spices, vegetables, and medicines. The fruit of Capsicum plants have a variety of names depending on place and type. They are commonly called chili pepper, red or green pepper, or just pepper in Britain and the US; the large mild form is called bell pepper in the US, capsicum in Australian English and Indian English, and paprika in some other countries.

The fruit of most species of Capsicum contains capsaicin a lipophilic chemical that can produce a strong burning sensation in the mouth (and, if not properly digested, anus) of the unaccustomed eater. Most mammals find this unpleasant; however, birds are unaffected. Apparently, the secretion of capsaicin is an adaptation to protect the fruit from consumption by mammals while the bright colors attract birds that will spread the seeds. The amount of capsaicin in peppers is highly variable and dependent on genetics, giving almost all types of peppers varied amounts of perceived heat. The only pepper without capsaicin is the bell pepper. Chile peppers are of great importance in Native American medicine, and capsaicin is used in modern Western medicine—mainly in topical preparations—as a circulatory stimulant and pain reliever