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Can you baffle your relatives this festive season with our 10 perspective-skewing stunts?

1. Aristotle illusion

What you’ll need:
Marbles, dried peas or just your nose

What to do:
Ask a friend or relative to cross their fingers and touch a small round object like a marble or a dried pea – it feels like touching two peas. This also works if they touch their nose. This is an example of what is called perceptual disjunction, which happens because our brains don’t account for the fact that the fingers are crossed. Because the pea (or nose) touches the outside of both fingers at the same time, the brain interprets it as two separate objects.

2. The two boxes trick

What you’ll need:
Two cardboard boxes – one large, one small – and two identical objects of the same weight, eg bricks

What to do:
Put a brick in each box and check the boxes are the same weight. Then ask someone to lift them and tell you which is heavier. The vast majority of people will say that the smaller box is heavier, even though it isn’t, and will continue to maintain that it is, even after looking inside both boxes and lifting them several times. This is an example of incorrect perceptual weight judgement.

3. Curved credit card confusion

What you’ll need:
A blindfold and a credit card or library card

What to do:
Blindfold a relative and, using a credit card, move it back and forth in a straight line along the pad of their index finger – it should feel straight. Now, repeatedly but slowly rotate the card in a see-sawing motion on their finger. They should now think that the card feels curved. This will work even if they do it to themselves. This is an illusion of proprioception.

4. Japanese illusion

What you’ll need:
Two hands

What to do:
Ask your friends and family to hold their arms out in front of them, cross them and rotate their hands so that their palms are facing. Then they should clasp their fingers together and bring the hands up between their arms, exposing the knuckles. Now ask them to move a specific finger. It’s tricky, and might take a few attempts to get it right.

5. Chalkboard illusion

What you’ll need:
A chalkboard, chalk and a pair of earplugs or noise-cancelling headphones

What to do:
Asksomeone to write something on the chalkboard using the chalk, then rub it out and write it again wearing earplugs (or, better still, noise-cancelling headphones). They should notice that the board feels much smoother when they can’t hear the chalk squeaking across its surface. This is an example of a cross-modal interaction, because what we feel is strongly affected by what we hear.

6. The sign of six

What you’ll need:
A desk or table and a chair

What to do:
Ask a friend of relative to sit at the desk or table and to lift their right foot off the floor and make clockwise circles with their leg. Now, while doing this with their foot, ask them to try to draw a number six in the air with their right hand, starting at the top. Often their foot will change direction. With practice, it is possible to overcome this tendency, much in the same way many have overcome the classic ‘pat your head and rub your stomach’ challenge.

7. The Pinocchio effect

What you’ll need:
Two chairs and a blindfold

What to do:
Place the chairs one in front of the other, both facing in the same direction. One person should put the blindfold on and sit in the rear chair, facing the back of the person sitting in front. Then ask the blindfolded person to reach around and put their finger on (not up!) the nose of the other person. At the same time, ask them to place their other hand on their own nose and to begin gently stroking both noses in a downwards motion. After about a minute, more than 50% of subjects say their noses feel incredibly long – hence the Pinocchio effect.

8. Motion after-effects 

What you’ll need:
A large circle of paper, string and a push pin

What to do:
Cut a spiral from a large circle of paper, then hang it from the ceiling so it spins constantly. Ask your friends and relatives to stare at the moving spiral for a few minutes and then to quickly look down at the back of their hands. They will appear to move in the opposite direction to the spiral. This is called motion after-effect. It works because the visual neurons that fire in response to the motion get fatigued. When we look at something else, these neurons fail to fire and our brains interpret that as movement in the opposite direction.

9. Magic elbow

What you’ll need:
A chair and an electric toothbrush

What to do:
Ask someone to close their eyes and touch the end of their nose. Then ask another person to turn on the toothbrush and place it in the crook of the first Scout’s arm at the end of the bicep. The vibrations stimulate muscles in the biceps that would normally be activated by the elbow straightening. However, because the fingers are still giving tactile information that they are touching the nose, the brain thinks that the nose is moving away from
the face.

10. Change blindness

What you’ll need:
Two images of almost identical scenes, but with one minor detail altered. Google ‘change blindness’ for some excellent examples that you can print out.

What to do:
Show your friends and family two scenes that are identical in all but one feature, interspersed with a brief blank screen, and they will almost certainly fail to notice the change. This is known as change blindness, which occurs because usually we have to be paying attention to a change to notice it. The brief blank screen acts as a mask that prevents the change from attracting attention.

  

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