Fire. The word itself sounds like fire—igniting the kindling sound ffff into a bright, airy blaze of vowel, the r at the end of the word curling like the tip of a flame. Fire. One of four elements—fire, water, air, soil—making up our historical description of this planet. Fire. A metaphor for ingenuity, creativity, intelligence, and passion. Fire. A little bit of sun right here on earth.

In the stories of Greek mythology, Prometheus steals fire from the gods and gives it to humankind. When he does this, we benefit from his gift in a lot of ways. Fire gives us the spark of heat and energy and the spark of creativity. Suddenly civilizations flourish. Technology is born. We cook food, construct houses, forge tools, invent machines. We expand our horizons from eking out a mere existence to creating ideas of mathematics, sciences, arts, and societies. The gift of fire forges us into the bright creatures we are.

So we can think of fire as an important aspect of our everyday lives. It’s easy to notice and imagine it all around: it’s in the sun, in a campfire and a stove, in a car engine. Fire is energy and it can make things go.

But what is fire exactly? What’s it made of? What causes fire? One way to start to answer that question is to look at how we use fire. In many applications, human beings use fire as a primary source of heat, burning fuels like wood. And in some applications fire is used to create another sort of energy altogether, as when a fire burns in an engine that in turn creates electricity. That’s a short explanation of what fire does, but it doesn’t exactly tell you what it is.

What does the dictionary say? Fire is “a rapid, persistent chemical change that releases heat and light and is accompanied by flame, especially the exothermic oxidation of a combustible substance.” That definition contains a few more words to look up in the dictionary! The first part makes sense because we’ve observed the phenomenon in the description. When you look at a fire burning in a fireplace, flames move quickly, flickering and popping. And a “combustible substance” isn’t tough to understand; it’s anything that burns. But “exothermic oxidation”? Okay. That phrase describes the process of something burning and combining with oxygen while releasing heat. It’s a mouthful, but this is the crucial part of the definition because these words really talk about fire’s transformation of fuel into energy. Fire is often said to “eat” things because it does something similar. Fire “eats” oxygen and combustible substances and then releases energy in the form of heat and light.

Maybe the next question you’re asking is: what’s the connection between fire and biomass? And hey—what’s biomass? Good question! Biomass is plant material, vegetation, or agricultural waste used as a fuel or energy source. Fire burns biomass for energy like electricity. When Prometheus brought fire to humankind, he conveyed it on a dried stalk of fennel, which burns very slowly and consistently. Burning that piece of fennel in the myth was the first utilization of biomass.

Biomass is an important part of human history, and it will be an important part of our future.