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Donate them. Some stores have donation boxes near the cash register to garner donations for organisations like seeing-eye dogs, UNICEF, etc.
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Make a purchase at a convenience store. Dump all your coins as there’s a coin counting machine that will determine whether you have paid enough.
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Go to a Pocket Change machine (https://www.pocket-change.jp/en/) and swap these loose change for gift cards.
- Use it as a resource for learning Science. These coins are made of purely aluminium. They are designed to weigh exactly 1g, so lightweight that people mistake them for being plastic. They are legendary for being so light that they can float on water by not breaking the surface tension. (It’s easier to achieve this feat though if you slowly put the coin in hard water like mineral or hot spring water.)