Here's why:

* Element: An element is made up of only one type of atom (e.g., gold, oxygen, hydrogen).

* Compound: A compound is formed when two or more different elements chemically combine in a fixed ratio (e.g., water, salt, carbon dioxide).

* Mixture: A mixture is a combination of two or more substances where each substance retains its own properties (e.g., salt and pepper, sand and water).

Water (H₂O) is formed by the chemical combination of two hydrogen atoms and one oxygen atom. Since it's made of two different elements, it fits the definition of a compound.