What is the easiest way to throw a ball and have it stop and completely reverse direction after travelling a short distance?
Throw it In the air.
How can you throw a ball as hard as you can and have it come back to you, even if it doesn't bounce off anything? There is nothing attached to it, and no one else catches or throws it back to you?
On every moving railway carriage there are metal parts which are travelling in the opposite direction to the train and others which are travelling at twice the speed of the train. What are they?
