Photons have mass? I didn't even know they were Catholic

This joke shines by cleverly playing on multiple meanings of the word "mass." The initial setup hinges on the scientific concept of mass, a fundamental property of matter, but then takes an absurd detour into the religious realm. By asking if photons are Catholic, the joke hilariously misinterprets the physical "mass" for the Catholic religious service, setting up a delightful pun that twists scientific inquiry into theological speculation.

The real-world context here involves photons, which are the elementary particles of light. In physics, a key characteristic of photons is that they have zero rest mass, making the premise "photons have mass?" a scientific misdirection (Review) that adds to the joke's charm for anyone with a basic understanding of physics. This scientific inaccuracy is precisely what allows the comedic misinterpretation to take flight.

The punchline, "That's a light subject," brings it all home with another layer of wordplay. "Light" refers both to the literal nature of photons (they are light!) and to the idiomatic phrase for a topic that isn't serious or heavy. This neatly ties back to the initial confusion about "mass" (heavy vs. light) and perfectly encapsulates the joke's clever, multi-layered linguistic gymnastics, turning a scientific concept into a wonderfully silly pun.