Biology

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Video

Materials

• Your nose

Explanation

The phenomenon of feeling two nasal tips when crossing the fingers was described by Aristotle (384-322 BC) in one of his writings. That's why this classic and fun demonstration has been named Aristotle's illusion.

Aristotle, however, had no explanation for the phenomenon. But one have emerged in modern times. The brain, through its physiology, is tuned to how the body is normally positioned. What happens when you touch the tip of the nose is that sensory cells both on the left side of the index finger and on the right side of the middle finger register something. This usually only happens when there are two different objects that the fingers touch, because these finger sides are so far from each other. So when the brain in this case receives signals from sensory cells from both of these parts of the body, it interprets it as two objects - even though it's only one.

It's unclear why the fingers are so sensitive to this phenomenon, when there are other body parts that can be crossed or placed in strange positions and whose sensory signals are still interpreted correctly by the brain.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you cross your middle and ring fingers instead?
• What happens if you cross your index and ring fingers instead
• What happens if you cross your index fingers from both hands instead?
• What happens if you cross your arms and touch an object in the "wedge" between them?

Variation

You can also place your crossed fingers against a pea, a marble or the edge of a table.

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