input: Write a python function to split a string into words.
  assert split_string("This is a test string") == ['This', 'is', 'a', 'test', 'string']
  assert split_string("Hello world") == ['Hello', 'world']
  assert split_string("Python is great") == ['Python', 'is', 'great']
output: def split_string(s):
      return s.split()