input: Write a function to classify animals as either "Bird" or "Mammal".
  assert classify("hummingbird") == "Bird"
  assert classify("elephant") == "Mammal"
  assert classify("cat") == "Mammal"
output: def classify(animal):
      birds = ["hummingbird", "sparrow", "eagle", "parrot", "penguin"]
      mammals = ["elephant", "cat", "dog", "tiger", "lion"]
  
      if animal in birds:
          return "Bird"
      elif animal in mammals:
          return "Mammal"
      else:
          return "Unknown"