input: def find_longest_word(sentence):
      words = sentence.split()
      # initialise longest_word variable with 0
      longest_word = 0
      for word in words:
          if len(word) > longest_word:
              longest_word = len(word)
              longest_word_str = word
      return longest_word_str
      """
      >>> find_longest_word("The rain in Spain stays mainly in the plain.")
      'mainly'
      """
output:     words = sentence.split()
      # initialise longest_word variable with 0
      longest_word = 0
      for word in words:
          if len(word) > longest_word:
              longest_word = len(word)
              longest_word_str = word
      return longest_word_str