input: Write a python function to find the longest word in a given sentence.
  assert longest_word("The quick brown fox jumps over the lazy dog") == "jumps"
  assert longest_word("Python is a powerful programming language") == "programming"
  assert longest_word("To be or not to be that is the question") == "question"
output: def longest_word(sentence):
      words = sentence.split()
      longest = max(words, key=len)
      return longest