input: Write a python function to split a string into a list of words separated by spaces.
  assert split_string_into_list("hello world") == ["hello", "world"]
  assert split_string_into_list("this is a test") == ["this", "is", "a", "test"]
  assert split_string_into_list("python is fun") == ["python", "is", "fun"]
output: def split_string_into_list(s):
      return s.split()