input: Write a python function to categorize a given word into predefined categories of fruits, vegetables, or animals.
  assert categorize("apple") == "fruits"
  assert categorize("carrot") == "vegetables"
  assert categorize("dog") == "animals"
  assert categorize("chocolate") == "unknown"
output: def categorize(word):
      fruits = {"apple", "banana", "orange", "grape", "mango"}
      vegetables = {"carrot", "broccoli", "spinach", "potato", "onion"}
      animals = {"dog", "cat", "elephant", "tiger", "lion"}
  
      if word in fruits:
          return "fruits"
      elif word in vegetables:
          return "vegetables"
      elif word in animals:
          return "animals"
      else:
          return "unknown"