input: Write a function to identify the longest word in a given sentence. If there are multiple words with the same maximum length, return the first one encountered. 
  
  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("Hello world") == "Hello"
  assert find_longest_word("Python programming is fun") == "programming"
  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