Magic Octagon – Solid Copper Geometric Brain Teaser & Magic Trick Coin

Two engraved arrows, one on each face, turn this solid copper coin into a geometric brain teaser. The arrows sit at a fixed 90° angle to one another, but you can see only one face at a time. Flip and rotate the coin, then try to predict which way the hidden arrow will point. Instructions are included.

The 1-inch (2.5 cm) coin weighs 16.3 grams and has a patina applied to highlight the engraving. Designed in collaboration with Raymond Hall (@physicsfun) and engraved by Shire Post Mint, it brings a longstanding geometric magic trick into a small copper format.

A Geometric Puzzle with a History

The first documented description of this brain teaser appeared as a magic trick by Elias et al. under the title Screwy Square, published in 1946. Variations have used a square or octagon with an arrow on each face, keeping the essential feature: the arrows are at a 90° right angle to one another.

In Over the Coffee Cups (1949), Martin Gardner described a variation called Crazy Crackers, which he credited to Val Evans. That version used a marking pen and a square soda cracker. Once the audience was astonished, Gardner advised, “crumble it up before anyone has a chance to experiment with it”. English magician Willane published The Chinese Compass in Willane's Wizardry (1947), establishing the octagonal presentation of the puzzle.

The challenge involves tracking the second arrow through three rotations: two vertical flips and one 45° rotation around a centered axis perpendicular to the coin's face. Although the arrows' apparent directions change as you handle the coin, their angle relative to each other stays fixed. Try making a prediction before each turn, then compare it with the revealed direction. This makes the coin a hands-on way to investigate rotations and the preservation of angles in a geometry discussion.

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Specifications

• Material: solid copper
• Size: 1 inch (2.5 cm)
• Coin weight: 16.3 grams
• Finish: patina to highlight the engraving
• Instructions included
• Designed in collaboration with Raymond Hall (@physicsfun); engraved by Shire Post Mint

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