input: Write a python function to reverse the words in a given string.
  assert reverse_words_in_string("hello world") == "olleh dlrow"
  assert reverse_words_in_string("python is fun") == "nohtyp si nuf"
  assert reverse_words_in_string("data science") == "atad ecneics"
output: def reverse_words_in_string(s):
      return ' '.join(word[::-1] for word in s.split())