input: Write a python function to split a string into words separated by spaces.
  assert split_words("Hello,World!") == ["Hello,World!"]
  assert split_words("This is a test string") == ["This", "is", "a", "test", "string"]
  assert split_words("   Leading and trailing spaces   ") == ["", "", "", "Leading", "and", "trailing", "spaces", "", "", ""]
output: def split_words(s):
      return s.split(" ")