Instructions:

Ask someone to hold out their arm and close their eyes.

Hold two pens together and put them tops facing down on one of their fingertips.

How many pen tops do they feel?

Still with their eyes closed, repeat the experiment on the person’s forearm.

How many pen tops do they feel this time?

Explanation:

Your fingertips are very sensitive, as they contain lots of mechanoreceptors. Your forearm is less sensitive as the nerve network is less dense there, which explains why you need to move the pens away from each other in order to tell them apart.

Studies on hands show that our fingertips can distinguish two points only 2 mm apart. On the palm, this distance is 5 to 10 mm and on the forearm, you need a gap of at least 40 mm to be able to distinguish between two points.

Note: The spatial discrimination threshold is the minimum gap required between the two points of dividers to be able to tell them apart. It varies according to the part of the body being stimulated.