When you bang a part of your body against something, your skin might turn different
colors where you bumped it. That splotch of purple, green, blue, and black is called
a bruise, and it's caused by blood leaking from broken blood vessels (the tubes that
contain blood) under your skin. Because you didn't cut yourself,
the blood from the damaged blood vessels can't come out and instead gets trapped under
the skin. A bruise may look icky, but your body will make it disappear in no time!