2013-05-06
Explanation
This comic plays on the concept of genetic similarity between humans. An alien named Mendax confesses to cheating on his human partner with another human named Robert. The alien's excuse is that humans share 99.9% of their DNA with each other, so from his alien perspective, he genuinely couldn't tell the difference between the two humans. The human partner, moved by the confession, forgives the alien and says "I still love you, Mendax."
The twist comes in the final panel when the alien calls the human "Hank," and the human responds "Who's Hank?" -- revealing that the alien has in fact confused this human with yet another human. The alien sheepishly says "I'll just let myself out." The joke works on multiple levels: it satirizes the common cheating excuse of "it didn't mean anything," it plays with the real scientific fact that humans are genetically near-identical to one another (which would indeed make us look interchangeable to a sufficiently alien observer), and it delivers a perfect comedic callback by having the alien immediately demonstrate the very problem he just described.