fossil fuel
A fossil fuel is sunlight from the deep past, bottled. Coal, oil, and natural gas all began as living things — ancient plants, plankton, and algae that soaked up sunshine, died, and got buried. Over millions of years, heat and the crushing weight of rock cooked their remains into black coal, dark crude oil, and invisible gas. Strike a match to any of them and you release energy that ancient life captured from sunlight — some of it long before the dinosaurs walked the Earth.
That ancient life was made of carbon, and so the fuel is too. When we burn it, that carbon joins with oxygen from the air and flies off as carbon dioxide (CO₂). The catch is timing: it took the Earth tens of millions of years to lock that carbon underground, and we are unlocking it in a couple of centuries. The CO₂ piles up in the air faster than the planet can absorb it, thickening the blanket of greenhouse gases that traps heat — which is the engine of today's warming.
A common mix-up: fossil fuels are not made from dinosaurs. The word 'fossil' just means 'dug up from the ground' — most of the buried life was tiny ocean plankton and swampy plants, not great reptiles. And these fuels are not running out tomorrow; the deeper problem is that we have far more than the air can safely take.
Burning carbon: C + O₂ → CO₂ — buried carbon meets air's oxygen and leaves as heat-trapping gas.
'Fossil' comes from the Latin fossilis, 'dug up' (from fodere, to dig) — the same root behind dinosaur fossils, though the fuels themselves are mostly the buried remains of ancient plankton and plants. Coal powered the steam engines of the 1800s; oil and gas took over in the 1900s, and together they still supply roughly four-fifths of the world's primary energy (about 80 percent, and slowly falling — though their share of electricity is lower, closer to three-fifths).