Put a solar panel on the robot. Use the controller for the robot to make it move around using the suns energy. Say this should be the use of energy for cars, making them run by electricity. If they ask about nighttime, give the example of a glow-in-the-dark magnet. It gathers the energy it needs during the day. uses it at night. Also use the example about the terrible oil spill in the gulf of mexico thats going on right now. Say that would be abolished if we were to use an alternative energy source for our cars and machines, aka the suns energy.Don't know much about enviormental engineering, but I think this falls into that catagory. Not very original but good examples for it are here.