The reason you should never trust an atom is a classic joke: "Because they make up everything." This is a pun playing on the dual meaning of "make up." Atoms "make up" everything in the physical world since all matter is composed of atoms, but "make up" also means to fabricate or lie, implying atoms are not trustworthy because they "make up" (fabricate) everything.
The joke is popular in science humor and often used to lighten discussions about atoms and matter.
