Modifying an element in a list in a function is the same as if you were just modifying an element of a list outside a function.

def double_first(n):
  n[0] = n[0] * 2

numbers = [1, 2, 3, 4]
double_first(numbers)
print numbers

1. We create a list called numbers.
2. We use the double_first function to modify that list.
3. Finally, we print out [2, 2, 3, 4]

When we pass a list to a function and modify that list, like in the double_first function above, we end up modifying the original list.