input: Write a python function to generate a response based on predefined responses.
  assert respond('hi') == "Hello! How can I help you?"
  assert respond('is it raining') == 'Yes, it is raining.'
  assert respond('will it rain') == 'It might rain, but I cannot predict the weather.'
  assert respond('unknown message') == "I'm sorry, I don't understand."
output: def respond(message):
      responses = {
          'hi': "Hello! How can I help you?",
          'is it raining': 'Yes, it is raining.',
          'will it rain': 'It might rain, but I cannot predict the weather.'
      }
      return responses.get(message, "I'm sorry, I don't understand.")