input: Write a python function to break a string into a list of words and convert them to lowercase.
  assert break_string("Hello World") == ["hello", "world"]
  assert break_string("This is a Test") == ["this", "is", "a", "test"]
  assert break_string("PYTHON programming") == ["python", "programming"]
output: def break_string(s):
      return s.lower().split()