Hydrogen may be the simplest atom, but it plays some of the biggest roles in the universe. With one proton and one electron, it is the most common element of all, making up about 75 percent of normal matter.

Hydrogen is the fuel of the stars. Inside the Sun, hydrogen atoms fuse to form helium, releasing the light and heat that reach Earth every day. Without hydrogen fusion, there would be no starlight, no warmth, and no life.

On Earth, hydrogen rarely appears as a free gas because it is so light it slips away into space. Instead, it is bound into water and organic molecules. Every drop of water is two parts hydrogen and one part oxygen. Hydrogen bonds hold water molecules together and give it its unusual properties, like surface tension and the ability to dissolve so many substances.

Hydrogen also runs through every living thing. It is part of proteins, fats, DNA, and more. In your own cells, hydrogen atoms are constantly being shifted around, their electrons passed along like tiny sparks.

This flow of hydrogen is what fuels the reactions that produce ATP, the molecule that keeps you alive and moving.