The riddle
Why Can't You Trust an Atom?
Make up, twice
"Make up" can mean to form or compose, as in a team made up of eleven players. It can also mean to invent, as in making up a story. The riddle uses both. Everything around us is made up of atoms, and a person who makes things up is not to be trusted. The joke works only because one phrase carries the two meanings at once.
How small an atom is
Britannica gives the size of an atom as roughly 10 to the power of minus 10 meters, a ten-billionth of a meter. Placed side by side, ten billion atoms would stretch about one meter. Atoms are built from protons, neutrons and electrons, and nearly all ordinary matter is made of them. A single grain of sand holds far more atoms than any person could ever count one by one.
A name that means uncuttable
The word atom comes from the Greek atomos, meaning something that cannot be cut. Scientists later found that atoms do have parts, which makes the name a small joke on its own. The riddle needs none of that history, but it is a good extra for a group that likes the topic. The periodic table now lists 118 elements, according to IUPAC, and each one is a kind of atom with its own number of protons. Hydrogen, the simplest, has one.
Using it in a classroom
The riddle suits a classroom break because it takes thirty seconds and ties back to a real fact. After the groan, ask the group what else is "made up" of something, and the second meaning will come up again in a new form.