At first glance, a 1-yen coin seems unremarkable, but this feather-light aluminum disc is a masterclass in physics and design.

Introduced in 1955, it weighs exactly 1 gram and spans just 20 millimeters. Made of pure aluminum, it’s so low in density that surface tension can hold it afloat on water—an everyday object turned into a physics demonstration. Like ships balanced on the ocean, the coin presses down, but the water’s molecular “skin” pushes back, keeping it afloat.

Place two of these coins side by side and something remarkable happens: they creep together. The scientific term for this is the Cheerios effect, where floating objects bend the water’s surface into tiny slopes, sliding toward one another as though pulled by a miniature gravity.

What began as Japan’s smallest unit of currency has also become a physics favorite, a humble coin that reveals the hidden rules of surface physics while carrying with it decades of economic history.

Includes 3 floating coins