Here's why:

* Fossil fuels are formed from the remains of ancient organisms (plants and animals) that lived millions of years ago.

* They are non-renewable, meaning they are formed over extremely long periods and are being depleted faster than they can be replenished.

* They are hydrocarbons, meaning they are primarily composed of carbon and hydrogen.

* They are combustible, meaning they release energy when burned.

While often grouped together, it's worth noting that each fossil fuel has a distinct origin:

* Oil is formed from the remains of microscopic marine organisms buried under sediment and exposed to heat and pressure over millions of years.

* Coal is formed from the remains of ancient plants that were buried under sediment and subjected to heat and pressure.

* Natural gas is often found alongside oil and is believed to form from the same organic matter as oil, but under different conditions.