What do you buy when you already have everything? Luxurious homes, private jets, and expensive schools have long been the preserve of the super-rich. But as the number of millionaires and billionaires increases, people are looking for other ways to show off their status – and the latest 'must-haves' are anything but conventional. Read on as we round up some of the strangest things the world's super-wealthy are spending a fortune on today, from moon rocks to multimillion-dollar digital art.