Watch this clip, and try and work out what he's saying. (No, it's nothing as complex as a whole sentence, just a sound)
Then watch it again without looking at it, and try work out what he's saying.
http://www.media.uio.no/personer/arntm/McGurk_medium.mov
Then watch it again, KNOWING what he's saying, and it'll STILL sound like it did first time.
SO weird.