Asked by: BBC Focus Magazine
Familiar from cartoon strips, this bizarre phenomenon is caused by the generation of so-called phosphenes – illusory flashes of light caused by pressure on the light-receiving cells in our eyes. The simplest – and least painful – way of generating them is by gently pressing on your closed eyelids then opening your eyes. Isaac Newton came up with another method while investigating the nature of light: sliding a bodkin (a think, blunt needle) between his eye and the socket-bone. Not recommended.