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

This joke shines by playing on two very different meanings of the word "mass." In the world of physics, a photon, the fundamental particle of light, is famously massless. It's a key characteristic that allows light to travel at its incredible speed. But then the joke takes a sudden, hilarious detour, implying "mass" as in the religious service, specifically a Catholic Mass. The humor comes from the unexpected shift from a scientific property to a religious ceremony, creating an absurd image of a photon attending church.

The punchline, "That's a light subject," cleverly doubles down on the wordplay. "Light" directly references the photon itself, the very subject of the joke. However, "light" also means something not serious or heavy, something easy to discuss. So, we're talking about a topic concerning light, which is also being treated in a "light," humorous way. It’s a neat, two-for-one pun that wraps up the scientific absurdity with a linguistic bow, making for a truly bright moment of comedy.