﻿ Article Title | Interact | physics.org

# Shrinking Coin

This trick requires the audience to think very much "outside the box". Most people simply won't believe it can be done – but with some 3D thinking it's easy!

## Ingredients:

• 1x 1p coin
• 1x 2p coin
• piece of paper (approx 10cm x 10cm)
• pencil
• scissors

## Instructions

1. Lay the 1p coin in the centre of the piece of paper. Trace around it using the pencil.
2. Cut out the centre of the circle so that you are left with a piece of paper with a hole in the centre.
3. Demonstrate that the 1p coin slips easily through the hole.
4. Challenge your audience to get the 2p coin through the 1p-sized hole – WITHOUT ripping the paper or altering it in any way. Give them some time to try (multiple coins & paper to distribute amongst your audience can help at this stage).
5. Show them how it can "really" be done:
1. Take the piece of paper and bend it in half. Hold the paper so that the bend is at the bottom. Drop the 2p coin between the sides of the paper into the centre of the hole.
2. Grasp the paper between finger and thumb near the bend, on either side of the coin. Slide your fingers upwards around the coin. Allow the paper to buckle outwards in the dimension perpendicular to the coin.
3. The coin should slip through the hole!

## How Does it Work?

This is all to do with non-Euclidean geometry. The small 2D hole may be stretched in the third dimension to produce a slit that is large enough to allow the larger coin through.

## Tips for Success

Don't use the same piece of paper too often or it will develop permanent folds in it, which can cause the coin to get stuck AND help your audience guess the solution to the trick!

## Serving Suggestions

This is a good hands-on activity for younger audiences, where they can all have a go at cutting out the hole and trying the trick. However, older audiences also won't believe that it's possible. A good trick to use once you have already formed an audience.

## Did You Know?

One of the more intellectually difficult aspects of string theory for the lay person is the concept of folded dimensions. If you are feeling brave, you might want to use this trick as a way to demonstrate how multiple dimensions can be folded into tiny spaces.