Why is snot green?

Asked by: Sam Evans, Poole

Healthy nose mucus is clear, but when you have a cold, white blood cells called neutrophils are sent to fight the infection. One of the bug-killing enzymes they produce is called myeloperoxidase, which has a heme (iron-containing) pigment that causes the green colour in secretions such as pus and nasal mucus.

Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.