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
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.
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.