input: Write a python function to split a string into words using space as the delimiter.
  assert split_string("The quick brown fox jumps over the lazy dog") == ["The", "quick", "brown", "fox", "jumps", "over", "the", "lazy", "dog"]
  assert split_string("Hello World") == ["Hello", "World"]
  assert split_string("A B C D") == ["A", "B", "C", "D"]
output: def split_string(s):
      return s.split()