Take a test with this video and see how you do: McGurk Effect.

What am I saying? Play the clip several times, alternating between looking at the talking head while listening, and listening with your eyes shut. Most adults (98%) think they are hearing “DA” – a so called “fused respons” – where the “D” is a result of an audio-visual illusion. In reality you are hearing the sound “BA”, while you are seing the lip movements “GA”. The “McGurk effect” was first described by Harry McGurk and John MacDonald in “Hearing lips and seeing voices”, Nature 264, 746-748 (1976).

Interesting demonstration.