Sometimes we hurt ourselves. When this happens, some of our blood vessels break, blood piles up under our skin and we see a bruise. These have the familiar ‘black and blue’ or purple appearance in the beginning, but gradually change to different colours. The purple is given by haemoglobin, a protein that carries the oxygen in our red blood cells. Our bodies react to this, with some white blood cells called phagocytes ‘eating up’ the materials in the bruise. As phagocytes degrade the haemoglobin, they turn it into other molecules. As you can imagine, different molecules show different colours, and the bruise will change colour with time to green, yellow and brown. When everything has been cleared up by your immune system the bruise disappears and you’re ready to bump into something else.