You are presented with the quiz "What converts electrical energy into light energy when it is turned on?" 

But you don't know the answer, so you turn to your teacher to ask for hints. He says that "alight bulb converts electrical energy into light energy when it is turned on" and "in Denver, traffic signal lights are being converted to the new energy-saving bulbs". 

So, what's the best answer to the question?


 

 Something found in a traffic signal          