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Potholes

In the upper part of a river's course, the water flows very swiftly and it can carry large pebbles.

If pebbles are swirled around in the same place, they rub against the bed and wear away a pit, known as a pothole. In ways like this, a river near its source cuts swiftly into its bed.

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Q1:The pebbles wear the bed away, but what happens to the pebbles as they rub against the bed?

[Answer]