Math π Mystery: planned 1-of-1 physical card

The π Mystery

Swedish: Talet pi · First estimate: Rhind Papyrus approximation, Egypt, about 1650 BC · Archimedes' bounds, 3rd century BC

Every circle, from a coin to a Ferris wheel, goes round exactly π times its width.

Why it works

Measure round a plate, then across it. Divide. You get a little over 3. Do it with a coin, a bike wheel, a round table: the same number every time. That number is π. (This holds for perfect circles on a flat surface. Planet orbits are ellipses, not circles.)

Its decimal digits never end and never settle into a repeating block: Johann Lambert proved in 1761 that π is irrational. People have calculated trillions of digits, so every circle hides a number nobody will ever write down in full.

The π card is the SparkON mystery card. The full product plan is one physical card only: brushed metal, with π in real gold foil and its digits running round the edge, hidden in one random booster pack. Serial-number registration on this site is planned and not available yet.

What it lets you predict

The way round any circle, from one straight measurement.

How tests ask about it

• Circumference and area of a circle
• Why π never ends

Try it

A bicycle wheel is 70 cm across. How far does the bike roll in one full turn of the wheel?

Connects to

Wins these Lab Duel challenges

• Not in the first challenge set yet.