On 23 May, the mathematician John Nash was killed in a car crash, alongside his wife Alicia. The couple were in their 80s. Professor Nash was on his way home from Norway after receiving the prestigious Abel prize for mathematics. He also won the Nobel memorial prize in economics in 1994, and was made famous far beyond academia when he was played by Russell Crowe in the film, A Beautiful Mind. Tim Harford takes a look back at his life with economist Peyton Young who knew Nash well. Tim also looks at how many species of owl there are. A much more difficult question to answer than you would think.