Forest
Stay focused and a tree grows; leave the app and it dies.
Forest is a focus timer with a single, memorable mechanic: start a session and a seed is planted; stay focused until the timer ends and it grows into a tree; leave the app early and the tree dies. Over time your sessions accumulate into a forest, and rewards unlock new tree species.
The writeup in the source article points at why this works: the IKEA Effect, our cognitive bias toward valuing things we made ourselves. The forest is something you grew, so abandoning a session has an emotional cost far beyond a broken counter.
The design lesson: one legible mechanic, deeply tied to the core behavior, beats a dashboard of points. Forest is the go-to example of gamification through a single metaphor.
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Longer write-ups covering this example and others like it:
