I have a small strawberry birthmark on my thigh. It didn't come out until my late teens. My mom has a larger one in the same place.
One time when I was in the sixth grade I walked into the bathroom to find the other girls sucking on their own and each others arms to make hickeys. It was a new discovery! I watched with interest at what they were doing.
One of the girls showed me the mark it left on her arm and I said, "that's nothing, my mom has a big one on her leg!" All the other girls lost it.
It took me a few years to figure out that the girls thought my dad had put a hickey on my mom's thigh!