Everyone knows what a coffee bean looks like: a small brown nugget filled with a deep aroma that transports us to Kenyan plantations. But place two or three beans side by side, not only from the Kenyan savannah but also from Colombia's evergreen forests or the islands of Sumatra, and you will find that although they are all coffee, the basic ingredient — the bean — is quite different. Size, shape, colour and, ultimately, the taste and aroma of a brew are characteristic of particular varieties, cultivars and sometimes regions. What actually causes these differences, and why can the seeds of the same plant be so different?