how-to-spot-a-physicist
Explanation
The Joke
Two museum workers are discussing a practical problem: how to detect when physicists enter the museum. One of them proposes a simple solution: put up a sign advertising an "Interactive Dark Matter Exhibit" and then watch for the one person in a thousand who starts giggling.
The bottom panel shows the sign posted outside the museum: "Today: Interactive Dark Matter Exhibit!"
The Humor
The joke relies on understanding what dark matter is. Dark matter is a hypothetical form of matter that, by definition, does not interact with electromagnetic radiation -- meaning it cannot be seen, touched, or detected through any normal interaction. An "interactive" dark matter exhibit is therefore a hilarious oxymoron: you literally cannot interact with dark matter. That is its defining property. The only people who would find this sign funny are those who know enough physics to recognize the contradiction, which is why only a physicist would start giggling at it. Everyone else would walk past it thinking it sounds like a perfectly normal museum exhibit.
References
Dark matter is a theoretical form of matter that is believed to account for approximately 27%% of the mass-energy content of the universe. It has never been directly observed because it does not emit, absorb, or reflect light -- it only interacts gravitationally. Its existence is inferred from its gravitational effects on visible matter, such as the rotation curves of galaxies and gravitational lensing.