input: Write a python function to join a list of strings into a single sentence.
  assert join_words(["hello", "world", "!"]) == "hello world !"
  assert join_words(["python", "is", "fun"]) == "python is fun"
  assert join_words(["this", "is", "a", "test"]) == "this is a test"
output: def join_words(words):
      return ' '.join(words)