Birthmarks get their name for one simple reason: they are marks that are present on the skin of a lot of newborn babies! A baby can develop birthmarks either before he is born (while he's still inside his mother's body) or soon after birth.

Most of the birthmarks that kids have are called hemangiomas (say: he-man-jee-oh-mas). These birthmarks happen when many new blood vessels group together in one place on the skin. Blood vessels are tiny tubes that carry blood through the body.

No one knows what causes many vessels to show up together, but it's good to know that most birthmarks don't hurt at all and they usually aren't a sign of any kind of illness. Lots of newborns have these birthmarks on the back of the neck (where they're called "stork bites") or between the eyebrows (where they're called "angel kisses"). These birthmarks usually disappear within the first few weeks or months of life.