[punching noise] Ow! That hurt. That’s going to be black and blue. Which raises a question…why does a black eye turn blue? and then green, and then yellow and then brown, finally, before it disappears? It’s all because of your hemoglobin; the compound in red blood cells that brings oxygen to your body. When you get hit, the blow crushes tiny blood vessels called capillaries. Red blood cells ooze out of the broken capillaries into the surrounding tissue. From the outside of your skin, this mass of cells looks bluish-black, which is where we get the term, “black and blue”. Once out of the capillaries, the red blood cells no longer function, so other cells, called phagocytes, move in to mop up the debris. They engulf the red blood cells and start to break them down. First, the phagocytes break down the hemoglobin into biliverdin, a kind of bile; this turns the bruise green. Then, the phagocytes break down the biliverdin into bilirubin, which is yellowish. Bilirubin is waste we excrete and gives urine its yellow color. Finally, the phagocytes break down the bilirubin into hemosiderin, which is basically the iron that was in the hemoglobin. It’s brownish and is slowly absorbed into the body. The whole colorful process can typically take two weeks and sometimes the bruise moves downward, just because gravity is pulling the debris in that direction. So, next time you’re showing off your nasty bruise or black eye and somebody says, “Ew, it’s turning green!” You can just say, “Hey, it’s my hemoglobin…well, it used to be my hemoglobin.” For Scientific American’s Instant Egghead, I’m Mark Fischetti.