Brilliant! An element cannot be broken down into other substances. Water is made up of Hydrogen and Oxygen. So, water cannot be an element.
In other words, elements are made up of only one kind of matter. For example, Hydrogen is an element. We know that Water can be split into Hydrogen and Oxygen, but, Hydrogen cannot be split into any other element. Hydrogen is not made up of anything else.

Elements are further classified into metals, non-metals and metalloids.