input: Write a python function to randomly select and print a joke from a predefined list.
  assert tell_joke() in jokes
  assert isinstance(tell_joke(), str)
  assert tell_joke() != ""
output: import random
  
  def tell_joke():
      jokes = [
          "Why don't scientists trust atoms? Because they make up everything!",
          "What do you get when you cross a snowman and a vampire? Frostbite.",
          "Why was the math book sad? Because it had too many problems.",
          "Why did the scarecrow win an award? Because he was outstanding in his field!",
          "Why don't skeletons fight each other? They don't have the guts."
      ]
      return random.choice(jokes)