Gently press a few slightly-spread fingertips into a friend's back, without telling them how many fingers you are using.

Ask them how many fingers you're using, and chances are they will get it wrong.

Why is that? Grab a willing volunteer and we'll find out...

Step 1: Equipment Required

Squeeze your earlobe. Squeeze it hard. That doesnt really hurt, does it? Now try squeezing just as hard at the top of your ear ... ouch!

The nerves in your skin are spread out differently in different parts of your body. This activity will show you just how differently.

You need a volunteer who is not too easily embarrassed (if they are feeling really cooperative, they could be wearing shorts & tee-shirt to expose as much polite skin as possible), two full-size pencils, an elastic band, a ruler, and an outline of a body. Any simple silhouette will do, as long as you can write on and around it.

Step 2: How to Measure the Sensitivity.

First, you need your test equipment. That's the pencils. Lay them blunt end to blunt end, over-lapping a couple of centimetres, and fasten them together by twisting the rubber band around them, as in the diagrams below.

The pencils can then be bent to make a sort of pair of tweezers.
Open the tweezers wide and touch them to the subject's skin when they have their eyes closed, and they can feel two points on their skin. Lift them off, squeeze the points slightly closer, and touch again. Can they still feel two points? Lift the tweezers off, squeeze some more and try again. Keep doing this, and eventually the subject will only be able to feel one point, even though two points are touching their skin.

Measure how far apart the pencil-points are, and record it on the silhouette. Try this over as much of the subject's skin as you can, front and back of the body (it will also work through a single layer of clothing, such as a t-shirt). Remember to respect your subject - do not map bits that they do not want mapped.

Step 3: Questions You Should Think About.

This is a Science Instructable, so we should really be asking questions about what we are doing and the results we get:

What do you notice?
Which parts of the skin are most sensitive?
Why do the least sensitive parts not need a very accurate sense of touch?
Repeat the experiment with several people (you should take your turn as a subject as well) - does everybody agree about their results?
Why do you think there are differences?
Why is it a good idea to test more than one person in an experiment like this?

Step 4: Recording Your Results.

There are several ways of recording and displaying the results. Colour-coding is good - compare each other's results and agree on which parts are most sensitive and which parts are the least sensitive, and agree on a colour-scheme to decorate a large outline (drawn around a subject?).

What is the best way to agree on sensitivity? Just talk it through? Or is there a mathematical way of doing it?

Step 5: Take Things a Bit Further...

.... and stab each other.

Use a very sharp pencil to gently prod all over the skin of your volunteer's hand. Sometimes they will feel nothing, sometimes they will feel a sharp pain, or a dull pressure, or even hot or cold. This is because you have different nerves to sense different things, and a sharp-enough pencil will hit a single nerve and make it send a signal to your brain.

Step 6: What I Have Not Done

I haven't told you exactly how to record your results.
I haven't told you how to display your results.
I haven't told you how to compare your results.
I haven't told you how to check your results.

Why not? Because if I told you exactly what to do, you wouldn't be doing proper capitalised Science. You wouldn't be thinking about what you are doing, you wouldn't be considering ways of making the experiment better as you go along. You wouldn't be coming to your own conclusions based on the evidence, rather than received wisdom (down that road lies true Folly).

Have a go, and post your results and conclusions.