It may come as a surprise to some of our male readers, but you all actually started out as females - physically and phenotypically speaking. So how is this possible? Well, we all know that when a man and a woman make a tiny human, they each provide 23 chromosomes. One pair of chromosomes helps to determine the baby's sex - two X chromosomes and it'll be a female and an X chromosome from the mother and a Y chromosome from the father and it'll be a male. The key here, explains the episode of AsapSCIENCE above , is that the Y chromosome doesn't kick in right away.