input: Write a python function to convert a word to Pig Latin.
  assert pig_latin("hello") == "ellohay"
  assert pig_latin("apple") == "appleay"
  assert pig_latin("smile") == "ilesmay"
  assert pig_latin("stair") == "staairay"
  assert pig_latin("Python") == "ythonPay"
output: def pig_latin(word):
      vowels = "aeiouAEIOU"
      if word[0] in vowels:
          return word + "ay"
      else:
          for i in range(len(word)):
              if word[i] in vowels:
                  return word[i:] + word[:i] + "ay"
          return word + "ay"