The "McGurk Effect" illustrates that what our eyes see can influence what we hear. The video here below shows Prof Patricia Kuhl's demonstration of this effect. She is mouthing the syllables /ga-ga/, but the video has been dubbed with a sound track of her saying /ba-ba/. Your eyes can tell that the lips are not closed at the beginning of the syllables, and they therefore tell your brain that the syllable cannot be /ba/, even though in reality it is. Trying to reconcile the conflicting information from your eyes and ears, the brain will decide that the syllables are those that is acoustically closest to /ba-ba/ which are articulated with the lips open, and you will "hear" /da-da/ or /tha-tha/. Howver, if you play the video again, but close your eyes, you will be able to convince yourself that the sound track is really /ba-ba/. Play it over and over, and open and close your eyes at random. The sound you hear will flip from /tha-tha/ to /ba-ba/ depending on whether your eyes are open or not.