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Nobel Prize winning psychologists, Christopher Chabris and Daniel Simons had a theory - They believed that humans focus only on things they want to - sometimes missing the most obvious stuff going on around them.
To verify their intuition, in 1999 they created a short video dubbed 'Invisible Gorilla', in which six people, half dressed in white shirts and the other half in black, were filmed passing basketballs around. The viewers were asked to keep a silent count of the number of passes.
Somewhere in between, a chest-thumping Gorilla entered the fray and stayed in the film for a full nine seconds. However, when asked, most people were so focused on counting that they totally missed the Gorilla!
We have all of course gotten much more observant since than, or have we? Check out the sequel to the 1999 experiment and see if you can catch every change that occurs in this short video. Be sure to tell us how you did, by adding your comments below.