input: Write a python function to split a string into a list of words.
  assert split_string("Hello, World!") == ["Hello,", "World!"]
  assert split_string("This is a test.") == ["This", "is", "a", "test."]
  assert split_string("Split this string") == ["Split", "this", "string"]
output: def split_string(s):
      return s.split()