# What Makes π Special

By Vabole

Topic: Maths

[Open the interactive Jig](https://ask.jig.com/j/Y20rNb90n4yX)

## Guide

### 1. A number in every circle

Every circle hides the same special number, called pi and written π\. It is a little more than 3, and it never changes\. Let's find it with wheels, plates, waves and pizzas\. Drag anywhere to turn the table\.

Mathematicians have used the Greek letter π for this number for more than 300 years\.

### 2. Across and around

The red rod goes straight across the wheel, through its middle\. That distance is the diameter\. The distance all the way around the blue tyre is the circumference\. Pi links these two\.

### 3. Roll it out

Roll the wheel exactly one turn and its inky tyre paints its circumference onto the board\. Then copies of the red diameter are laid along the line\. Three fit, with a small yellow piece left over\.

The leftover is about 0\.14 of a diameter\. That is why π is about 3\.14\.

### 4. Any size, same answer

Make the wheel bigger or smaller\. The painted line and the rods grow and shrink together\. It is always three diameters plus the same small piece\. That never\-changing amount is π\.

A 26\-inch bike wheel rolls about 2 metres in one turn: π times its width\.

### 5. Archimedes' trick

Over 2,000 years ago, Archimedes trapped π between two shapes with straight sides\. The orange one sits inside the circle and the green one outside\. Give them more sides and both squeeze closer to the circle\.

With 96 sides, he proved π is between 3\.1408 and 3\.1429\.

### 6. Circles make waves

As the crank turns, its yellow bead rises and falls, and the arm draws that height as a wave\. One full turn draws one whole wave, as long as the circle's circumference\. Sound, light, radio and mains electricity travel as waves like this, so π appears wherever we work with them\.

Engineers often measure a full turn as two π, about 6\.28\.

### 7. Pizza puzzle

The big pizza is twice as wide as each small one\. How many small pizzas hold as much? Four\! Pi links a circle's width to its area, the space it covers, so doubling the width gives four times the pizza\.

A 16\-inch pizza has about as much as four 8\-inch pizzas\.

### 8. Digits that never end

Pi's digits go on forever and never settle into a repeating pattern\. No fraction can ever equal it exactly\. In 2025, computers worked out 314 trillion digits, just for the challenge\.

NASA's engineers use only 15 decimal places of π to steer spacecraft across the solar system\.

### 9. Spot π today

Pi helps people build wheels and gears, size pipes and cans, steer spacecraft and send music through the air\. Next time you see a circle, remember the same number is hiding inside it\.

Pi Day is celebrated on 14 March, written 3/14 in the United States\.