The Bulgarian conceptual artist Christo has wrapped both the Reichstag in Berlin and the Pont-Neuf bridge in Paris in fabric. He also built a nearly forty kilometre fabric fence running down to the sea in California. At the age of 81, Christo is still working. His latest piece is a vast floating pier - again draped in fabric - on Lake Iseo in northern Italy that allows people to walk on water. On board a boat on the lake, Christo talks about his latest project.