- Moving something is a kind of function
- Webbed feet are used for moving faster through water by aquatic animals
- Hence, the function of webbed feet is to move aquatic animals through water faster

- Moving faster is a kind of method for moving better
- Helping means something can be done better
- Therefore, helping something move can mean causing something to move faster

- The function of webbed feet is to move aquatic animals through water faster
- Helping something move can mean causing something to move faster
- Therefore, the function of webbed feet is to help aquatic animals move through water

- The function of webbed feet is to help aquatic animals move through water
- Swimming is when animals can move in water
- Hence, the function of webbed feet is to help aquatic animals swim

So, the function of webbed feet is to help aquatic animals swim
Question: The main function of a duck's webbed feet is to help the duck
Context: - Swimming is when animals can move in water 
- Moving something is a kind of function 
- Moving faster is a kind of method for moving better 
- Webbed feet are used for moving faster through water by aquatic animals 
- Helping means something can be done better