input: Write a python function to split a sentence into words.
  assert split_sentence("Hooray! Great!") == ["Hooray!", "Great!"]
  assert split_sentence("The quick brown fox jumps over the lazy dog") == ["The", "quick", "brown", "fox", "jumps", "over", "the", "lazy", "dog"]
  assert split_sentence("Python is fun") == ["Python", "is", "fun"]
output: def split_sentence(sentence):
      return sentence.split()