Watch a mouth say 'ga' while you hear 'ba', and your brain invents a third sound

In 1976 psychologists Harry McGurk and John MacDonald dubbed the audio 'ba' onto video of a mouth saying 'ga' and found that most viewers heard neither: they perceived 'da'. The illusion shows that speech perception isn't just heard, it's seen. Closing your eyes makes the 'ba' return instantly.

— Harry McGurk, John MacDonald, Hearing Lips and Seeing Voices — Nature, vol. 264, 1976