input: Write a python function to return the first characters of each word in a string.
  assert first_characters_of_words("This is a test string") == "Tiat"
  assert first_characters_of_words("Hello World") == "HW"
  assert first_characters_of_words("OpenAI creates amazing tools") == "Ocat"
output: def first_characters_of_words(s):
      return ''.join(word[0] for word in s.split())