Here's why:

* Atoms are the smallest unit of an element that retains the chemical properties of that element. They are the fundamental particles that make up everything around us, from the air we breathe to the water we drink.

* Atoms combine to form molecules. Molecules are groups of two or more atoms held together by chemical bonds. These molecules can be simple, like a water molecule (H₂O), or complex, like a protein molecule.

* Substances are made up of molecules. A substance is a pure form of matter with a definite and consistent composition. For example, water is a substance made up of water molecules.

So, while molecules and substances are important, it's ultimately the atoms that are the building blocks of everything.