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.