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A ball of green algae that grows on the floor of a few cold lakes. The best known ones live in Lake Akan in Hokkaidō, Japan, where they have been a protected Special Natural Monument since 1952 and have their own festival each autumn. They grow only a few millimetres a year, and they are given as a charm. The name comes from mari, a bouncing ball, and mo, a water plant.

No. If you neglect it, it turns pale and looks tired — but it never dies and there is no illness to cure. Take care of it again and it recovers on its own.

Yes. The text of your wish stays on your device. It is never sent to a server, never used for anything, and your marimoshi never reads it or refers to it — it only tells you that it is holding it. It is also left out of the image when you share.

One card each morning. It opens when your care average is 50% or more; below that your marimoshi tells you it can’t give you one today. The card is drawn from its mood, the season and the days you have spent together, and you can share it. It is for entertainment only.

After seven different days of care you earn a sprout, and one more every seven days after that, up to three waiting at a time. A sprout can’t be used on yourself — it is only ever given away. Whoever receives it starts with that sprout as their own marimoshi, and it keeps your name on it.