Many of our body parts come in pairs - eyes, ears, hands, feet, and so on. In rare instances when the rule of two is exceeded, the result is unusual to say the least. One of the more well-documented exception is the nipple - the third nipple, to be more specific.
Third nipples are known medically as supernumerary nipples. Although these can occur anywhere on the body, they more commonly occur along the body's "milk lines" - two vertical lines that run from each underarm, through the nipple, to the groin.
The presence of more than two nipples is considered rare, affecting less than 0.5% of Americans (around 200,000). However, some people argue the condition may be more common since third nipples don't usually look like a fully formed nipple and may be easily confused with moles.
If you do find a 'nipply'-looking mole along your mammary lines, it could indeed be a third nipple. And if so, you'd share a commonality with Mark Wahlberg, Lily Allen, and Tilda Swinton!