Show children a simple optical illusion, such as a picture that appears to be of either a young girl or an old woman (see Resources section).
Ask children what they see in the picture. Wait until you hear both possible responses -- old woman and young girl.
Explain the illusion. Run your finger or a pointer along the line of the young girl. Then run your finger along the line of the old woman. You might wish to create two distinct images colored to emphasize either the young girl or the old woman.
Describe the nature of the illusion. The young girl / old woman illusion is an ambiguous illusion. This is one that creates a perceptual "switch" in which an observer sees two interpretations.
Show children other examples of ambiguous illusions to enhance their understanding.
Explain distorting illusions. Display a distorting illusion, such as the crazy line trick (see Resources section). Ask children to explain what they see. After children describe their perceptions of the illusion, explain that the image actually consists of lines that are straight, not crooked.
Cover a section of the image to enhance the straight appearance of the lines. Explain that a distorting illusion forces the eye to incorrectly judge the shape, size or length of an image. Show other examples of distorting illusions to the children.
Provide an example of a paradox illusion, such as the Penrose triangle (see Resources section). Explain that image is of an impossible object. The Penrose triangle creates an image that bends in an impossible manner. Describe the reasons for paradoxical illusions -- that people hold certain untrue beliefs about objects. In the Penrose triangle, the untrue belief is that the triangle's edges must connect. Display other paradoxical illusions to illuminate the point.