Capsaicin is the substance that makes chili peppers hot/spicy/piquant.  Here are some interesting facts about capsaicin (obtained from Floridata):

  • The capsaicin in chili peppers can burn the skin and irritate the eyes.
  • Capsaicin is not soluble in water and is difficult to wash off with soap and water.
  • Unlike mammals, birds are immune to the effects of capsaicin.
  • Most of the capsaicin is located in the interior tissue of the pepper, where the seeds are attached. Peppers can be rendered considerably less piquant by slicing in half and scraping out the seeds and associated membranes with a spoon.
  • The amount of capsaicin varies among chili pepper types, with the growing conditions (hotter, dryer weather makes for hotter peppers) and even between fruits on the same plant. Ripe peppers are sweeter and often (but not always) hotter than green peppers.
  • Peppers are sometimes ranked in Scoville Heat Units (SHU) which are nothing more than the number of times an extract of pepper dissolved in alcohol can be diluted half and half in sugar water and the capsaicin still tasted. Bell peppers get a zero on this scale; jalapeños around 3,000; tabascos around 60,000; and habaneros around 300,000.