input: Write a function to identify the longest word in a sentence. Use the following function header:
  def find_longest_word(sentence):
      words = sentence.split()
      longest_word = ""
      for word in words:
          if len(word) > len(longest_word):
              longest_word = word
      return longest_word
  
  assert find_longest_word("The rain in Spain stays mainly in the plain.") == "mainly"
  assert find_longest_word("Python programming is fun and exciting!") == "programming"
  assert find_longest_word("What is the longest word?") == "longest"
  assert find_longest_word("I like to code") == "like"
  assert find_longest_word("") == ""
output: def find_longest_word(sentence):
      words = sentence.split()
      longest_word = ""
      for word in words:
          if len(word) > len(longest_word):
              longest_word = word
      return longest_word