Energy
People have used energy to cook and for heat for thousands of years. Over time we have learnt to get energy from a number of different sources, but today we mainly rely on energy from 'fossil fuels': coal, oil and gas. Fossil fuels were created more than 300 millions years ago, by trees and plants that died sinking into swamps and being compressed into rock, and eventually into coal, oil or gas. These materials are extracted from the earth and used to provide energy to power factories, heat our homes, fuel our cars and so on. They now supply more than 90% of the world's energy. However, the fossil fuels in the ground will eventually run out.
Coal
One of the main uses of coal is to make electricity. Electricity is made in power plants where coal is burned to change water to steam, and the steam turns big wheels called turbines. In poorer countries coal is still important for household heating and cooking. Coal is by far the most abundant fossil fuel, found in over 100 countries.
Oil
Crude oil is separated into different products at an oil refinery, including petrol, diesel fuel, kerosene, tar and chemicals to make plastic. More than half of the world's oil is found in the Middle East.
Gas
Natural gas (mostly methane), is mainly used for heating homes and buildings, heating water and cooking. Before use it is sent to a factory, for the methane to be separated from the other gases. It creates fewer greenhouse gases than coal or oil when it is burned.
There are other problems, not just running out of fuel. These fuels are also really dirty and when they are burnt they create pollution and release 'greenhouse gases' (such as carbon dioxide) into the atmosphere. Our demand for energy is now so high that the burning of fossil fuels is releasing enough greenhouse gases into the atmosphere to begin to directly affect our climate system. See Climate change.
We need to make sure that we use energy as efficiently as possible, and aim to lead a less energy-intensive lifestyle. Rich countries consume far more energy than poorer countries, which means they also produce far more emissions. However, everyone suffers because atmospheric pollution can travel anywhere, and climate change gases have a worldwide impact.