Some substances, like particles of this iron, contain only one kind of atom. Iron is an element. It is made of only one type of atom: iron atoms. Sulfur is another element. It contains only sulfur ...

Every element is made up of its own type of atom. This is why the chemical elements are all very different from each other. Everything in the universe contains the atoms of at least one or more ...

That pun is based on the fact that the main building blocks in the branch of science are chemical elements, pure substances ...