input: Write a python function to extract words from a sentence.
  assert extract_words("Hello world") == ["Hello", "world"]
  assert extract_words("Python is great") == ["Python", "is", "great"]
  assert extract_words("Write a function to extract words") == ["Write", "a", "function", "to", "extract", "words"]
output: def extract_words(sentence):
      return sentence.split()