Final answer:
The individual units that make up substances like water and methane are called molecules, which are formed when two or more atoms are bonded together by chemical bonds.
Step-by-step explanation:
The individual units making up substances such as water and methane are called molecules. A molecule is the smallest part of a substance that maintains the chemical properties of that substance. For instance, a molecule of water (H₂O) consists of two hydrogen atoms and one oxygen atom bonded together, and a molecule of methane (CH₄) has one carbon atom bonded to four hydrogen atoms. Molecules are the fundamental components of chemical compounds and are composed of two or more atoms joined by strong forces known as chemical bonds.