This free course was written by Sarah Vicary and Alun Morgan.
Figure 1 :Photograph: © Paul Pitman in Flickr
Figure 2: (top left); sturti/Getty Images ; (top right): monkeybusinessimages/Getty Images; (bottom left): Credit:monkeybusinessimages/Getty Images (bottom right): Eva Katalin Kondoros/Getty Images
Figure 3: Sasi Ponchaisang/123RF
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