- The grand canyon is a kind of canyon
- Most canyons are formed by flowing rivers through erosion over long periods of time
- So, the grand canyon is formed by flowing rivers through erosion over long periods of time

- A river is a kind of flowing body of water
- The colorado river is a kind of river next to the grand canyon
- Therefore, the colorado river is a flowing river next to the grand canyon

- The grand canyon is formed by flowing rivers through erosion over long periods of time
- The colorado river is a flowing river next to the grand canyon
- So, the grand canyon was formed by the flowing colorado river over long periods of time

- The grand canyon was formed by the flowing colorado river over long periods of time
- If something is formed by something else, then that something else is required by that something
- So, forming the grand canyon requires the flowing colorado river

- Forming the grand canyon requires the flowing colorado river
- If something requires something else then that something else is important to that something
- Hence, the flowing colorado river is important for forming the grand canyon

- A river is a kind of flowing body of water
- The colorado river is a kind of river
- Hence, the colorado river is a flowing body of water

- The flowing colorado river is important for forming the grand canyon
- The colorado river is a flowing body of water
- Hence, a flowing body of water is important for forming the grand canyon

Hence, a flowing body of water is important for forming the grand canyon
Question: Moving water was the most important factor in forming which of these?
Context: - The grand canyon is a kind of canyon 
- A river is a kind of flowing body of water 
- The colorado river is a kind of river next to the grand canyon 
- If something requires something else then that something else is important to that something 
- The colorado river is a kind of river 
- If something is formed by something else, then that something else is required by that something 
- Most canyons are formed by flowing rivers through erosion over long periods of time