But wait, aren't you a a super for real and not a lie relationship with some guy who is too embarrassed to reveal it here because he knows he'll get shit on if he actually exists?
It is a birthmark but looks like a scar. I can't tell you why it changed because it would spoil things.
But, this is why I asked if you wanted the spoiler.
There was a really nice pop book in my house when I was growing up will all kinds of scientific and biological information in it designed to get kids interested in stuff like this. Not to mention I learned this in grade school science.