Death is a central theme in Hirst's works. He became famous for a series of artworks in which dead animals are preserved in formaldehyde. The animals included a sheep, a cow and a shark. The best-known of these was "The Physical Impossibility of Death in the Mind of Someone Living". This was a tiger shark immersed in formaldehyde in a transparent display case. He has also made "spin paintings". These were created on a rotating canvas. And "spot paintings", which are rows of randomly coloured circles created by his assistants.