On a warm evening in East Texas, the edge of a field can start to sparkle. Tiny lights blink on and off above the grass, rising and drifting. These are fireflies, also called lightning bugs. Despite both names, they are not flies at all. They are beetles.

A firefly makes light inside its own body. This is called bioluminescence, which means light made by a living thing. In the lower part of a firefly's body, special cells hold a chemical called luciferin. When luciferin combines with oxygen and with a helper chemical called an enzyme, it gives off light. The firefly can control how much oxygen reaches those cells, which lets it switch its light on and off.

Firefly light is sometimes called "cold light." An old-fashioned light bulb wastes most of its energy as heat, so it can burn your fingers after it has been on for a while. A firefly turns nearly all of the energy from its chemical reaction into light, so its glowing body stays cool.

Why blink at all? For most fireflies, the flashes are a kind of language. Each species has its own pattern of flashes. In many species, males flash a signal as they fly, and females waiting in the grass flash an answer. If the answer comes at the right moment, the male knows he has found a mate of his own kind.

Young fireflies, called larvae, live on or in the ground, and many of them glow, too. They hunt snails, slugs, and worms.

Many people say they see fewer fireflies than they did as children, and scientists are studying why. Bright lights at night may make it harder for fireflies to see each other's signals, and the loss of fields and damp, grassy places leaves them fewer places to live. Some families now turn off their porch lights on summer nights and leave a patch of yard unmowed to give fireflies a chance.