Fireflies emit light through a process called bioluminescence. This remarkable phenomenon occurs when a chemical reaction takes place in the firefly's abdomen. The reaction involves the enzyme luciferase, the substrate luciferin, oxygen, and adenosine triphosphate (ATP). The emitted light is typically in the yellow-green spectrum and serves various purposes, such as attracting mates or deterring predators. The efficiency of this process is incredibly high, with almost no heat produced, making fireflies an excellent example of nature's ability to produce light.