You will be given a definition of a task first, then some input of the task.
Based on the information present in the given passage, you need to write a correct answer to the given question. A correct answer is one that correctly and completely answers the question. Do not write answers that address the question partially or incorrectly. The correct answer must be contained in the given paragraph.

Paragraph- Sent 1: Abrasion is another type of weathering.
Sent 2: Like ice wedging, it is a form of mechanical weathering.
Sent 3: Like ice wedging, abrasion does not change the rock into another type of rock.
Sent 4: With abrasion, one rock or piece of sediment bumps against another rock.
Sent 5: Rocks dont normally roll around on their own, so why do they move?
Sent 6: There are a couple of reasons a rock may move.
Sent 7: Gravity can cause rocks to move.
Sent 8: They may roll downhill or fall off a cliff.
Sent 9: As they roll down a hill, they bump into each other.
Sent 10: Maybe a moving rock hits a rock that is not moving at the bottom of the hill.
Sent 11: Moving water causes rocks and sediment to move.
Sent 12: Rocks are bounced along the bottom and bump against one another.
Sent 13: As they collide, they begin to chip away at each other.
Sent 14: Angular rocks become more rounded with each collision.
Sent 15: Strong winds cause abrasion.
Sent 16: The wind carries sediment.
Sent 17: This sediment is thrown against other rocky surfaces by the wind.
Sent 18: It is like sand-blasting a rock.
Question: How does gravity cause rocks to move?.
Output:
They may roll downhill.