input: Write a function to classify words into animals and feelings.
  assert classify_words(['happy', 'puppy', 'joyful', 'dog', 'cat']) == (['dog', 'cat'], ['happy', 'puppy', 'joyful'])
  assert classify_words(['sad', 'angry', 'cat', 'dog', 'joyful']) == (['cat', 'dog'], ['joyful'])
  assert classify_words(['elated', 'kitten', 'dog', 'happy']) == (['dog'], ['happy'])
output: def classify_words(words):
      animals = {'dog', 'cat', 'puppy', 'kitten'}
      feelings = {'happy', 'joyful', 'sad', 'angry', 'elated'}
      
      classified_animals = [word for word in words if word in animals]
      classified_feelings = [word for word in words if word in feelings]
      
      return (classified_animals, classified_feelings)