Watching lips say 'ga' while hearing 'ba' makes you hear a third sound

When the sound of one syllable is dubbed onto video of a mouth saying a different one, most people perceive a third syllable that matches neither the audio nor the video alone. It shows how strongly vision shapes what you 'hear' — close your eyes and the illusion vanishes, replaced by the real audio.

— Harry McGurk & John MacDonald, Hearing Lips and Seeing Voices — Nature, 1976