input: 
  from typing import List
  
  def common_elements(list1: List[int], list2: List[int]) -> List[int]:
      """ Finds and returns the common elements in two given lists.
      >>> common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])
      [21]
      """
  
output:     common = []
      for num in list1:
          if num in list2:
              common.append(num)
      return common