monaro

To prove they're mammals, obviously

It seems that human embryos develop mammary tissue before they bother to check on whether they're going to be male or female and start modifying the basic plan with surges of this or that hormone. After only a few weeks, milk ridges form -- two stripes of tissue that start in the armpits, curve out over the chest, go straight down the stomach and then veer in toward the groin, ending somewhere high on each thigh. Later the milk ridges regress to some extent, usually leaving us with just two nipples.

Quite a few people end up with an extra, or supernumerary nipple somewhere along the trail of the milk ridge, however. (One man had five.) Sometimes they can't be mistaken for anything but a nipple, and other times they look like a mole. In fact, many people with supernumerary nipples don't know they have them until some officious and informative person starts examining their moles. Extras often run in families -- Darwin cites two brothers who each had a supernumerary nipple. Anyone who thinks that's weird should immediately leave the room and go check his or her torso for moles. How do you know you're not head-to-foot extra nipples and we've all just been too polite to mention it?