I like optical illusions, such as the Ponzo Illusion, which is one of the better explanations of why the full moon on the horizon seems so big. They provide nice examples of the complex interplay of visual sensations and brain activity and cautions us that what we ‘see’ may not be really what is out there.
Here’s another really one called ‘Rotating Snakes’. Notice how different parts seem to rotate depending on the direction of your gaze.
Carl Zimmer provides the explanation for this illusion.