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A Touch of Deception



You and Your Students!

Directed By

Vicki Cobb, Education World Science Editor


Touch two noses on your own face and discover something about perception.


  • Human Body

Required Props


Setting the Scene

This simple activity can be done simultaneously by all students. It produces a wonderful teachable moment for introducing the sense of touch and the nervous system.

Stage Direction

This activity can be done when youre waiting for something else to happen. It fills up time in an entertaining way. Note: Some students will make a mistake and cross the wrong fingers. Watch for mistakes, and show them how to do it correctly.

Show-Biz Science is scripted by popular children's book writer Vicki Cobb. Click to learn more about Vicki or to read a brief synopsis of her philosophy of teaching science.

Visit our archive of archive of Show-Biz Science Activities. Watch for a new activity each week. Then chat with Vicki -- share your feedback and ask your questions about teaching science -- on our special Showbiz-Science message board.

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Act I
Aristotles Illusion
Aristotle was the first to name the five senses. All the senses provide information (send information to the brain), but the sense of touch offers the ultimate test of reality: if one can feel something, it was real.

However, the sense of touch can be fooled. The following illusion is called "Aristotles Illusion because it involves the sense of touch.

Invite students to cross the middle finger of their favorite hand over the index finger, a symbol some might use if they were going to tell a lie.

Then have them turn the palm of their hand over so they can look at it. Instruct each student to place the center of where her/his fingers cross on the tip of the nose. Then direct them to rub their crossed fingers up and down, underneath their nose near the nostrils and back up. Have students experiment with speed and pressure of their motion. Do their bodies will lie to them? They should feel two noses!

Act II
A Race to the Brain
Tap your two index fingers together and think about the sensations in your fingertips. Do you feel it more in one finger than the other? (You shouldnt. They should feel about the same.) Now tap a fingertip to your lip. Where is the sensation strongest? (The lip)

See below for the explanations of these two activities.

Behind the Scenes

Aristotles Illusion explained: When something from the outside world causes nerves to fire, its called "sensation. Light causes sensation in your eyes, sound in your ears, and pressure or temperature changes cause sensation in your fingertips. The message travels from the nerve endings to your brain at 80 miles per hour. When the brain gets the message, it interprets it. That is called "perception, and it is caused by a variety of factors including sensation; but it also involves your past experience. Since you have always perceived messages from your fingertips with your fingers uncrossed, sensations from crossed fingers are misinterpreted as two noses. Since you know this is ridiculous, you laugh. The word illusion means to mock, or to make fun of.

The Race to the Brain explained: When you tap your finger to your lip, the sensation message leaves both fingertip and lip at the same instant. Since the message from the lip has a shorter distance to the brain, it wins the race! The message that arrives first becomes the dominant sensation. Since both arms are presumably the same length, the fingertip-to-fingertip race ends in a tie; there is no dominant sensation. What if you were to touch a fingertip to a bare toe? Which will feel the dominant sensation? (The fingertip should win because it is closer to the brain.) What would happen with the toe to the lip? There are other factors at play here, including the density of sensory nerve endings. Some parts of the body have more than others. But experimenting with that is for another day.

The End

For more ideas for teaching about the sense of touch, see Vicki Cobbs book Feeling Your Way.

Article By Vicki Cobb
Education World®
Copyright © 2006 Education World