input: Write a function that takes two lists as input and returns a new list containing only the common elements. If there are no common elements, the function should return an empty list.
  assert common_elements([1, 2, 3, 4, 8], [2, 6, 8, 10])==[2, 8]
  assert common_elements([1, 3, 5], [2, 4, 6])==[]
  assert common_elements([1, 1, 1, 1], [1, 1, 1])==[1, 1, 1]
  assert common_elements([], [1, 2, 3])==[]
  assert common_elements([1, 2, 3], [])==[]
output: def common_elements(list1, list2):
      return [item for item in list1 if item in list2]