Why do men have nipples? The answer is simple: because they used to be female.
Now lads, before you spit out your pint of Guinness and drop your 'not for girls' Yorkie bar, rest assured that this decidedly feminine period took place while you were still in the womb (and pssst it happens to every man).
As the ASAP science video above explains when a man and female reproduce they combine their genetic information, providing 23 chromosomes each.
One set of these 23 pairs determines your sex: women have the XX chromosomes, while men have XY.
But, while gender is decided at the moment of conception, the Y chromosome found in males does not start to emit testosterone until around five to six weeks into pregnancy.
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So for the initial period all embryos follow a "female blueprint" - which involves developing everything from reproductive organs to nipples.
Once the Y chromosome kicks in it overpowers some elements of the X chromosome - such as descending ovaries to become testes and fusing labia to become scrotum. But by the time it gets to work the nipples have already been formed, they just will not turn into breasts.
Fascinating, non?