input: Write a python function to simulate a chatbot interaction and return the appropriate response based on the predefined responses.
  assert respond("what's your name?") == "My name is Chatbot."
  assert respond("what's the weather today?") == "It's sunny today."
  assert respond("unknown message") == "Default message"
output: def respond(message):
      responses = {
          "what's your name?": "My name is Chatbot.",
          "what's the weather today?": "It's sunny today."
      }
      return responses.get(message, "Default message")