Asked by: Dale Wilkinson, Guildford
Pressure on the eyeball compresses the retinal artery. You don't have to press terribly hard to cut off the blood supply to the retina entirely.
Deprived of oxygen, the photoreceptors can start firing randomly, causing swirling colours or flashing lights. It's a similar phenomenon to the stars you see when you stand up too suddenly.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.
