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When the Explainer asked you to vote on a favorite unanswered question of 2012, the majority opted for a rather lascivious query regarding why rich ladies sunbathe topless, and the Explainer has duly delivered your pound of flesh. But in perusing the runners-up, another question so intrigued the Explainer that he could not resist answering it as well: When and how did humankind figure out that sex is what causes babies?