input: Write a python function to classify animals based on their weight and size.
  assert classify_animal(5, 40) == 'small'
  assert classify_animal(15, 60) == 'large'
  assert classify_animal(8, 60) == 'medium'
  assert classify_animal(12, 45) == 'medium'
output: def classify_animal(weight, size):
      if weight < 10 and size < 50:
          return 'small'
      elif weight >= 10 and size >= 50:
          return 'large'
      else:
          return 'medium'