Most coins sink like stones when dropped in water. But Japan's humble 1 yen coin? It floats. This silver-colored disc, measuring just 20mm across, is made entirely of pure aluminum—making it so light that it weighs exactly one gram and can literally ride on water's surface tension.

That precision isn't accidental. Since its current design debuted in 1955, the 1 yen has maintained its exact 1-gram weight, turning it into an impromptu measurement tool. Need to weigh something small? Japanese households and laboratories have used these coins as makeshift gram weights for decades.

The Physics of a Floating Coin

The floating trick isn't magic—it's science. When placed carefully on still water, the 1 yen coin doesn't break through the surface tension. Water molecules cling together at the surface, creating an invisible "skin" strong enough to support the lightweight aluminum disc.

This makes the coin a favorite in science classrooms worldwide. Teachers use it to demonstrate:

Surface tension – how water molecules create a cohesive film
Density – aluminum is denser than water, yet buoyancy principles still apply
Buoyancy – objects can float if they displace enough water relative to their weight

Try to recreate the experiment yourself, though, and you'll discover the catch: it requires a gentle touch and completely still water. Drop it carelessly, and it sinks immediately.

An Endangered Species

Ironically, this engineering marvel is slowly disappearing from circulation. Between 2011-2013 and again since 2016, Japan's mint stopped producing 1 yen coins for general use, minting them only for collector sets. The culprit? Electronic payment systems and digital wallets have made the tiny denomination largely obsolete.

The coin costs more to produce than its face value, and with cashless transactions dominating Japanese commerce, fewer people carry these aluminum discs in their pockets. What was once everyday currency has become a physics demonstration tool and a curious relic of precise Japanese engineering.

So if you find yourself with a 1 yen coin, you're holding a small marvel: legal tender, precision weight, and floating novelty all in one.

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