All the life we ever see around us on Earth is powered by light from our local star, the Sun - either directly or indirectly. But where does the Sun get its power from?
Like all stars, our sun was born in a cold, cold cloud of gas and dust, of the sort that fills much of the space between stars. If some part of these clouds gets denser than its surroundings, usually because it's squished by a star exploding nearby, its gravitational attraction starts to pull in more and more matter, eventually forming a big ball.
All the stuff falling in to the centre squashes it together and causes it to heat up, delivering energy in the same way that anything does when it falls from a great height. If it all gets squashed enough and hot enough, hydrogen atoms start to fuse together. That releases even more energy, again for almost the same reason things release energy when you push them over a cliff - atomic nuclei are strongly attracted to each other once they get close enough, even though there are other forces working to keep them apart until they get there. Once they do get the chance to come together, the energy of their intense attraction is released as they unite.
When the hydrogen in the young star starts fusing into helium, the added heat and pressure make it easier for more hydrogen to fuse, and the process sustains itself until the hydrogen is used up. If the star is big enough, it can also fuse heavier elements together for even more energy, and keep on going a lot longer.
So almost all the energy we use here on Earth - all of our food, and most of our electricity - comes from what was left over when a vast gas cloud collapsed in on itself, and the energy that collapse then extracted from pairs of atoms collapsing into each other.
This writeup was read by Jet-Poop for the E2 Podcast Season 6, Episode 2.