The Science Museum holds over 300,000 items, including such famous items as Stephenson's Rocket, Puffing Billy (the oldest surviving steam locomotive), the first jet engine, a remake of Francis Crick and James Watson's model of DNA, some of the earliest remaining steam engines, a working example of Charles Babbage's Difference engine (and the latter, preserved half brain), the first prototype (draft model) of the 10,000-year Clock of the Long Now, and documentation of the first typewriter. It also contains hundreds of interactive exhibits. A recent addition is the IMAX 3D Cinema showing science and nature documentaries, most of them in 3-D, and the Wellcome Wing which focuses on digital technology. 