The punchline lands with all the precision of a perfectly balanced chemical equation. "Periodically" works on two levels here - the obvious reference to the periodic table and the temporal meaning. Just like how I only tolerate my grad students' enthusiasm in measured intervals. The cat professor, with those ridiculous glasses and bow tie, represents all of us who've spent enough time in the lab to develop a deeply specific sense of humor that exactly 0.079% of the general population will appreciate. That's the atomic mass of gold, by the way. Which is what this joke is.
Periodically Brilliant Humor

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