Answer: Peppers, as we generally refer to them, are members of the Capsicum species, many of which are not hot or pungent. Two examples are the bell peppers grown in the US, as well as some paprikas grown in Europe. These varieties are used fresh, and many times used to enhance other foods with their colors. The wilder members of the Capsicum species, on the other hand, range from mildly to extremely pungent. This is due to the substance capsaicin, or, actually, a group of similar substances called capsaicinoids.
The capsaicinoids are unique compared to other spicy substances, such as piperine (black pepper) and gingerol (ginger) in that capsaicin causes a long-lasting selective desensitization to the irritant pain, as a result of repeated doses of a low concentration or a single high concentration dose. The result is an increasing ability to tolerate hotter foods.
So, in a nutshell, it's the concentration of the chemical capsaicin that makes a pepper hot.
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