the-truth-about-centaurs
Explanation
The Joke
A woman approaches a centaur with a question. She observes that the centaur has an entire horse body instead of legs, but its head is the same size as a regular human head. She then makes a pointed logical deduction: since the centaur has a much larger body to operate but the same brain size as a human, and since a significant portion of brain capacity must be devoted to motor control of that larger body, the centaur must actually be significantly less intelligent than a human.
In the final panel, the centaur yells to its companions: "She's on to us! Run! Run!" -- confirming that the woman's theory is correct and that the centaurs have been trying to hide their diminished intelligence.
The Humor
The comic takes the mythological concept of a centaur and applies real neuroscience to it with devastating results. The humor lies in the logical inevitability of the argument: if you really did have a human brain controlling a horse-sized body, you would need far more neurons dedicated to motor control and proprioception, leaving fewer for higher cognitive functions. The centaur's panicked, unsophisticated response ("She's on to us! Run!") perfectly confirms the hypothesis -- they are not smart enough to formulate a counter-argument and instead resort to fleeing, much like an animal would. The joke also plays on the idea that centaurs have been running a long con, pretending to be as smart as humans, which makes their exposure funnier.
References
Centaurs are creatures from Greek mythology with the upper body of a human and the lower body of a horse. They appear in many myths, including the battle of the Lapiths and Centaurs. The neuroscience behind the joke is sound: in real animals, a significant portion of the brain (particularly the cerebellum and motor cortex) is devoted to controlling the body, and animals with larger bodies relative to brain size do tend to have less capacity for complex cognition.