input: Write a function to find the common elements between two given lists. The function should return a list of elements that are present in both input lists.
  
  def find_common_elements(list1, list2):
      common_elements = []
      for i in list1:
          for j in list2:
              if i == j:
                  common_elements.append(i)
      return common_elements
  
  assert find_common_elements([3, 9, 15, 21, 27], [21, 7, 13, 29, 31])==[21]
  assert find_common_elements([1, 2, 3, 4], [3, 4, 5, 6])==[3, 4]
  assert find_common_elements([10, 20, 30], [40, 50, 60])==[]
  assert find_common_elements([], [1, 2, 3])==[]
  assert find_common_elements([5, 10, 15], [10, 15, 20])==[10, 15]
output: def find_common_elements(list1, list2):
      return list(set(list1) & set(list2))