PH
Think of any light source you're familiar with - light bulb, fire, flashlight. They all get hot when emitting heat, which becomes extremely obvious if you've ever tried to change a light bulb shortly after it was on.
Fireflies, however, have found a way to light up their rear ends without emitting (much) heat. This type of light is called bioluminescence. In the case of fireflies, it's achieved through a combination of oxygen, calcium, adenosine triphosphate, and luciferin in the presence of the enzyme luciferase. The result is a "cold" light, in which the critter glows without producing much heat.
This, of course, is a matter of survival; if fireflies produced heat along with their light in the same manner as a light bulb or fire, they wouldn't survive for long.
(c) Ranker