input: Write a function to merge two strings into a list of words that appear in both strings. Return the list of words.
  def string_to_list(s1, s2):
      s1_list = s1.split()
      s2_list = s2.split()
      output_list = s1_list + s2_list
      return output_list
output: def string_to_list(s1, s2):
      s1_list = s1.split()
      s2_list = s2.split()
      return list(set(s1_list) & set(s2_list))