The heat or hotness comes almost entirely from Chili. The scale of hotness is measured by the Scoville scale. It is a measure of the pungency (hotness) of chili peppers. This pungency is because of the presence of Capsaicin. Capsaicin is a chemical compound that stimulates our chemoreceptor nerve endings in the skin, especially the mucus membranes and provides us the sensation of heat. The number of Scoville heat units indicates the amount of present.
Please do share with us if you have tried any of the really hot chili peppers.