Why can't you trust the atom? 'Cause they make up everything.
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Explanation
Explain Bear
Listen here, dummy. The joke is a pun. "Make up" has two meanings: to compose or invent something (like a lie), and to be the basic components of something. Atoms are the basic components of everything, so the joke plays on this double meaning to say atoms are untrustworthy because they "make up" everything in both senses.