input: Write a function that finds the common elements in two lists and returns them as a list.
  assert common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31]) == [21]
  assert common_elements([1, 2, 3, 4, 5], [4, 5, 6, 7, 8]) == [4, 5]
  assert common_elements([10, 20, 30], [30, 40, 50]) == [30]
  assert common_elements([], [1, 2, 3]) == []
  assert common_elements([1, 2, 3], [4, 5, 6]) == []
output: def common_elements(list1, list2):
      return list(set(list1) & set(list2))