What makes Elon Musk tick?

Elon Musk, the space pioneer and electric car guru, now ranks as one of the world's richest men, thanks to a surge in the value of shares in his company Tesla. In an interview from 2014, he tells the BBC's chief environment correspondent Justin Rowlatt what drives him to take on some of the world's most technologically challenging projects, and how he feels about the wealth he's accumulated over the years.