Explain SMBC — the wiki for Saturday Morning Breakfast Cereal

A professor at a lectern poses a rhetorical question about what ears and noses have in common. He explains that evolution has adapted these sensory organs to be large enough to take in a great deal of sensory information, but small enough that nobody would attempt to stick his penis in them.

The caption at the bottom reads: "Evolutionary fun fact: There is a fundamental constraint on the size of sensory orifices." The joke takes a legitimate-sounding observation from evolutionary biology -- that there are tradeoffs and constraints on the size of body openings -- and adds an absurd, crude "constraint" that no biologist would ever seriously propose. It parodies the style of pop-science explanations that confidently present evolutionary "just-so stories" to explain anatomical features, while simultaneously being a straightforward dirty joke.