The reason blood is red is because it's got iron in it. If you look down a microscope at blood, what you'll see are thousands of tiny little red cells that are referred to as bi-concave discs. If you look at them from the side, they look like a number eight and that's because they've got a thick ring round the edge and a flattened centre, a bit like a doughnut. They're crammed with a substance called haemoglobin, and haemoglobin is what carries oxygen around the body. Haemoglobin's a protein - in fact it's four proteins stuck together - and in the centres of each of those four proteins there is an iron (Fe) atom, which gives haemoglobin its red hue. But blood isn't exclusively red. Some drugs can bind to haemoglobin and alter the chemistry of the iron atom, causing the blood to change colour. In fact, a sulphur-containing migraine treatment called sumatriptan can do this in some people, making their blood go green.

Also, many animals have a different form of the haemoglobin protein with a different metal at the centre in place of the iron, which can also alter the colour. The horseshoe crab, for instance, has blue blood because it uses copper (Cu) instead of iron. And the real hippies of the haemoglobin world are a kind of marine annelid worm. It actually has purple blood!