input: Write a function that takes two lists as input and returns a new list containing only the common elements.
  
  def common_elements(list1, list2): 
      common_list = []
      for element in list1: 
          if element in list2: 
              common_list.append(element) 
      return common_list
  
  assert common_elements([1, 2, 3, 4, 8], [2, 6, 8, 10]) == [2, 8]
  assert common_elements([1, 2, 3], [4, 5, 6]) == []
  assert common_elements([], [1, 2, 3]) == []
  assert common_elements([1, 2, 3], []) == []
  assert common_elements([1, 2, 3], [3, 2, 1]) == [1, 2, 3]
output: def common_elements(list1, list2): 
      common_list = []
      for element in list1: 
          if element in list2: 
              common_list.append(element) 
      return common_list