Playing at sight and playing from memory

Tom Service on two of the most astounding musical skills, which the majority of professional classical musicians have in abundance - the ability to play from memory, and the ability to play at sight, without study or much in the way of rehearsal. How and why do they do it?

With pianist and teacher Richard Sisson, and violinist Eva Thorarinsdottir, of the Aurora Orchestra, whose members are unusual in that they often play from memory as an ensemble.