Here's why:

* Compound: A compound is formed when two or more different elements chemically combine in a fixed ratio. Water is made up of two hydrogen atoms and one oxygen atom (H₂O).

* Element: An element is a pure substance that cannot be broken down into simpler substances by chemical means. Hydrogen and oxygen are both elements.

* Mixture: A mixture is a combination of two or more substances where each substance retains its own properties. Water is not a mixture because the hydrogen and oxygen atoms are chemically bonded together.

Let me know if you have any other questions!