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Rivers can turn rough rocks into smooth, round pebbles.

Rivers can turn rough rocks into smooth, round pebbles.

Rocks tumble and bump along in the moving water for a long time. Slowly their sharp edges wear away and become smooth.

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Sand is made of tiny bits of rock and shell. Sand is made of tiny bits of rock and shell. Volcanoes make brand-new rock when their lava cools. Volcanoes make brand-new rock when their lava cools. Some rocks have sparkly bits inside called crystals. Some rocks have sparkly bits inside called crystals. Sand can be many colours, even white, pink, or black. Sand can be many colours, even white, pink, or black. Diamond is the hardest natural material on Earth. Diamond is the hardest natural material on Earth.