What you see is often affected by what you expect to see.
When your brain expects to see one thing and is presented with something quite different, you may feel some peculiar sensations.
Note: This activity can be done with a glass mirror (12-inch glass mirror tile is commonly available in home improvement stores), but we highly recommend using plastic mirror because it is much safer. If you must use a glass mirror, tape the edges to prevent cuts and glue one entire surface to a backing of cardboard or wood to minimize shards flying if the mirror cracks or breaks. It's also not practical to drill a hole in a glass mirror, but you can use epoxy, Gorilla Glue, or another strong adhesive to glue the handles to the glass.
Square piece of acrylic plastic mirror, available from plastics stores (mirror shown in the photo is 10 inches [25 centimeters] per side, but exact size is not critical)
Two wooden dowels, 1 in (2.5 cm) in diameter × 1 foot (30 cm) long
Dowel screw (double-ended screw), 3/16 in x 3 in works well
Two drill bits, one slightly larger than the dowel screw and one slightly smaller
Use the larger drill bit to drill a hole in the middle of the mirror.
Use the smaller drill bit to drill a hole straight into one end of each of the wooden dowels (the dowels will be your handles).
Insert the double-ended dowel screw in the mirror hole and screw on the dowels (one on either side of the mirror) until tight.
To Do and Notice
Grab the dowel with both hands, one on each side of the mirror, so that your right hand is on the reflective side of the mirror, and your left hand is on the non-reflective (back) side of the mirror. While looking at the reflection of your right hand in the mirror, move your left hand, the one that is out of view behind the mirror.
Place the mirror on a table so that it is standing upright and hold your right arm so its reflection looks like a continuation of your left arm. Simultaneously snap the fingers on both hands several times, then stop snapping the fingers on only one hand. Have someone drop an object into your left hand while only looking at the reflection of your right hand. Experiment with other movements of your hands while looking at the reflection of your right hand in the mirror.
What's Going On?
When you look at the reflection of your hand in the mirror, your brain is fooled into thinking that the image it sees in the mirror is actually your other hand. When you move that hand, your brain naturally expects to see the hand move. After all, messages from the nerves in that hand tell your brain that the hand is moving. The hand’s apparent failure to move can be profoundly disturbing to your brain, as the nerve impulses don't match the information from your eyes!
Some amputees who experience phantom limb pain in their amputated limbs have successfully reduced their pain using a kind of mirror therapy that uses the same principles demonstrated by this experiment.
Mirror therapy is also used to help stroke patients recover function in their limbs, particularly the hands and arms.