The human mind is highly complex and there are lots of experiments you can do to explore its behaviour. The following activity will encourage your students to think about how the brain interprets information.
The development of computer software that enables your students to manipulate images makes the first activity easy and fun. However, it can easily be done with paper and scissors.
To create this illusion you need an image of a face. Carefully cut out a rectangle around the mouth and another around the eyes. Remove the pieces and put them upside down. Replace them in the spaces and stick them into position. The image is somewhat unpleasant. Now turn the whole image upside down. You should find that it looks better this way.
Introduce your students to this image when it is upside down. Now turn the image the correct way up. Your students should be quite surprised (even if they have seen the effect before.) They can try this using images from magazines, digital photographs, or pictures taken with disposable cameras.
The scientist who came up with this process was called Peter Thompson. He first tried it with a picture of the then prime minister Margaret Thatcher.
The illusion is known as the Thatcher effect.
Our brains are used to processing information about upright faces, as we see far more faces in this position than we do upside down. It is thought that the brain interprets the information from the upside down face in a series of blocks - the mouth, the eyes etc. In isolation, these seem fine, as they accord with our experience of the upright face. It is only when we turn the face the correct way up that we notice how odd the face looks.
Sophie Duncan is project manager for science at the BBC www.bbc.co.ukscience