Key concepts
The five senses
The nervous system
Neuroscience

Introduction
Are you ticklish? If so, you've probably noticed that some parts of your body are more ticklish than others. That's because your skin's touch receptors aren't evenly distributed—some areas have more and others have a lot less. In this activity, you'll learn more about your sense of touch by testing your body's own reactions.

Background
This activity is based on a physical exam used in medical checkups. Doctors use this test—called two-point discrimination—to study sensitivity on different parts of a patient's body. This sensitivity is pretty important—without it you might not notice when your finger gets too close to a flame or when you've scraped your knee. Thanks to your skin receptors, you can feel a soft fleecy blanket, a cold splash of water or a sharp pinch—ouch!

Skin receptors are actually tiny cells called sensory neurons. When your skin makes contact with something—say a fluffy bunny—sensory neurons receive that "fluffy" signal and send out their own signals that travel through your body to your brain, which puts them all together to recognize that you are feeling a fluffy bunny.

Materials
Paper clips
Ruler
Pencil and paper
A partner
Glass with ice cubes (optional)

Preparation
•    Bend a paper clip into a U-shape with the tips about one centimeter apart
•    Keep the glass with ice cubes in the freezer until the end of the activity

Procedure
•    Tell your partner to close his or her eyes—no peeking until the end of the test!
•    Explain that in this activity, you will touch different parts of your partner's hand and arm using a paper clip, and your partner will need to say whether he or she is feeling one point or two.
•    Gently touch both tips of the paper clip to the back of your partner's hand. Both paper clip tips should touch the skin at the same time. Do you think your partner can feel both ends of the paper clip?
•    Ask your partner if he or she feels one point or two. If your partner feels only one point, bend the paper clip U a little bit wider and repeat.
•    Record the smallest distance between paper clip tips where your partner feels two points. The smaller the distance, the more sensitive the area.
•    Repeat each of the previous steps, this time touching your partner's palm; then try them on a fingertip, elbow and arm, and so forth. Is your partner's sensitivity greater on the arm or elbow? Right or left hand? Palm or fingertip?  You can try this experiment on other parts of the body, too—like the knee, back, toe or cheek. Why do you think some parts of the body are more sensitive than others?
•    Switch roles with your partner and find your skin's best-sensing spots. Are your results the same? Is your palm more sensitive than your partner's? What about elbows?
•    Extra: You can also try putting an ice cube on a spot like the back of your partner's hand for 10 seconds. Then use the paper clip to check whether that area is more or less sensitive. Do you notice a difference? If so, why do you think there's been a change?

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