In this short film, Dr Bryan Lovell, President of the Geological Society of London, tells us about his favourite rock: the Hertfordshire puddingstone. He explains how puddingstone is not only interesting geologically (see this paper by Dr Lovell), but is also important for our study of the Earth's climate (as Dr Lovell explains in this article). The film was made for the Science Museum's 'Atmosphere' gallery (© Board of Trustees of the Science Museum 2010) and can be viewed here.