It runs and runs

It runs and runs,

yet does not run away;

it longs for sunlight,

and yet never sees it.

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This one seems like it is going to be one of those "road" riddles, but then it goes in a very different direction, with the poor thing longing for light but forever in the darkness. It's very sad in a sense, but perhaps also hopeful. There are other riddles like this about how the heart never stop beating, how we never stop breathing, etc.

The heart.