input: Write a Python function to classify an animal into one of four classes.
  assert classify_animal(True, True, True) == 'Reptile'
  assert classify_animal(True, True, False) == 'Bird'
  assert classify_animal(True, False, True) == 'Fish'
  assert classify_animal(False, False, False) == 'Mammal'
output: def classify_animal(has_scales, lays_eggs, lives_in_water):
      if has_scales:
          if lays_eggs:
              if lives_in_water:
                  return 'Reptile'
              else:
                  return 'Bird'
          else:
              return 'Fish'
      else:
          return 'Mammal'