As regular readers of this blog will know, I love optical illusions. Not only are they fun to watch, the can provide insights into the subtle operations of the brain and how it influences our perceptions of reality. The same applies to auditory input: as the video on the McGurk Effect shows, you can't always trust what you hear.
Some of the commenters at the Bad Astronomy blog said the McGurk Effect didn't work for them, but for me it worked ever time. I could even close and open my eyes during the "fa" speech and "hear" the change over and over.