Snot plays a powerful role in protecting us from disease – and its colour alone can provide insights into what's going on in our bodies. In Ancient Greece, snot was thought to be one of the four bodily fluids responsible for balancing human health and personality. The physician Hippocrates developed a theory stating that phlegm, blood, yellow bile and black bile were the four "humours". A person's balance of these humours could dictate their temperament; an excess of any of them could cause illness. (BBC News)