input: Write a function to find common elements between two lists and return them as a list. The function should iterate through both lists to find matching elements.
  assert find_common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])==[21]
  assert find_common_elements([1, 2, 3, 4], [5, 6, 7, 8])==[]
  assert find_common_elements([10, 20, 30], [20, 30, 40])==[20, 30]
  assert find_common_elements([], [1, 2, 3])==[]
  assert find_common_elements([1, 2, 3], [])==[]
output: def find_common_elements(list1, list2):
      return [item for item in list1 if item in list2]