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. 

"In fact, the first five to six weeks of embryonic development are attributed to the X chromosome alone, and females grow from embryo to fully developed through the influence of only the X chromosome," they say.

When it comes to males, after that five- or six-week period, a gene called the SRY gene will activate on the Y chromosome, and actively inhibit certain features of the X chromosome. It will also impose, through genetic dominance, male physiological traits such as the testicles. This means that if the SRY gene is not activated, the female phenotype and physical appearance - which means clitoris instead of penis - will persist.

And what about nipples, I hear you ask? Nipples form before the activation of the SRY gene, during those fateful five or six weeks, which means we all get nipples, but only the females end up with breasts attached to them. You learn something every day when the AsapSCIENCE boys are around.