Looking for a fun Pi Day activity? You can use wheels of different sizes to roll out the simplest mathematics activity that demonstrates the value of pi, or use an optional method to avoid using ink or dye. The purpose of this Pi Day activity is to show that the value of pi is the constant ratio of circumference to diameter.

But first, what and when is Pi Day?

## What is Pi Day?

Pi Day celebrates the number pi every March 14, or 3.14, as a mathematical pun on the first three digits of pi.

Pi is the ratio of the circumference of a circle to its diameter. The Greek letter pi, ‘π’, is the symbol used in mathematics. The formula is C = π*d = 2*π*r, where:

- ‘C’ is the circumference of a circle.
- ‘d’ is the diameter, or distance across the circle passing through its center.
- ‘r’ is the radius from the center to the circumference.

The decimal expansion of pi does not end, but the value of pi is approximately 3.14159. Pi is an irrational number because we cannot calculate it as the ratio of two integers, although it is fairly close to 22/7 = 3.142857142857142857…, a repeating decimal, rational number.

## How to Introduce Pi as a Ratio

The basic activity for introducing pi is to roll a wheel for exactly one complete revolution. Divide the distance covered by the diameter of the wheel to calculate the value of pi. It’s easy to see that one revolution of the wheel covers exactly the distance of the circumference, so long as the wheel rolls rather than skids. This demonstration makes an important point by using wheels with different diameters: the ratio is the same regardless of the diameter of the wheel.

## The Materials for Rolling Out Pi

The materials needed to roll out pi are:

- Note paper or log book
- Several different sizes of wheels, which may include a pizza cutter, wheels on toy cars, or coins
- One long sheet of paper more than three times the diameter of the largest wheel
- A ruler with fine markings
- A pencil
- Non-toxic and non-permanent ink or dye
- A calculator

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