介绍:
Rocks
This is a mountain.
A mountain is made of rock.
This is a boulder.
It broke off the mountain.
This is a stone.
It broke off the boulder.
This is a pebble.
It broke off the stone.
This is sand.
It broke off the pebble.
This is dust.
Dust is the tiniest piece of rock.
Dust is everywhere.
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