input: Write a function to classify animals based on their types.
  assert classify_animal("lion") == "mammal"
  assert classify_animal("eagle") == "bird"
  assert classify_animal("crocodile") == "reptile"
  assert classify_animal("salmon") == "fish"
  assert classify_animal("frog") == "unknown"
output: def classify_animal(animal):
      animal_types = {
          "lion": "mammal",
          "eagle": "bird",
          "crocodile": "reptile",
          "salmon": "fish"
      }
      return animal_types.get(animal, "unknown")