Cross your fingers, then touch a small spherical object such as a dried pea: it feels like you are touching two peas (Image: Stuart McClymont / Stone)

Read about all the tactile illusions in our special feature

One of the oldest tactile illusions is the Aristotle illusion. It is easy to perform. Cross your fingers, then touch a small spherical object such as a dried pea, and it feels like you are touching two peas. This also works if you touch your nose.

This is an example of what is called “perceptual disjunction”. It arises because your brain has failed to take into account that you have crossed your fingers. Because the pea (or nose) touches the outside of both fingers at the same time – something that rarely happens – your brain interprets it as two separate objects.

A variation on the Aristotle illusion is to cross your fingers, close your eyes and then touch two different objects simultaneously – a piece of Blu Tack and a dried pea, say – one with each fingertip. You will need someone to guide your fingers onto the objects, and the illusion doesn’t always work, but if you’re lucky your sense of touch will tell you that the objects are the opposite way round from where they actually are. This is because your brain fails to correct for the fact that your fingers are crossed over.

There’s also the reverse Aristotle illusion: cross your fingers and touch the inside of a corner of a room or a box. This time, because the wall is contacting the insides of your fingertips, you should feel one surface, not two. Some people even experience three. …