genera
Explanation
The comic shows what appears to be a nature documentary or casual conversation scene. A woman is holding something and says: "Aaah! A second red line's just appeared on my pregnancy test-looking thing!" Someone responds: "Wait, I think it's a... line, it's..." In the final panel, someone bursts in declaring: "IT'S A MATHEMATICIAN!" -- as if identifying a species.
The joke plays on multiple layers. First, there is a bait-and-switch where what looks like a pregnancy test scenario turns into something else entirely. The phrase "a real-valued function that's continuous on a closed interval and differentiable on the open interval" (or similar mathematical language visible in the panels) is the formal definition from real analysis, likely referencing the Mean Value Theorem or similar concepts.
The punchline "IT'S A MATHEMATICIAN!" treats the mathematician like a wildlife species being identified in the field -- as if spouting formal mathematical definitions is a diagnostic behavioral trait, the way a birdwatcher might identify a species by its call. The comic plays on the stereotype that mathematicians are compulsively precise and cannot resist correcting or formalizing informal statements. The nature-documentary framing ("genera" in the slug refers to biological classification) reinforces the joke that mathematicians are being taxonomically classified as a distinct species, identifiable by their distinctive vocalizations about real-valued functions and differentiability.