A railroad operator lived with his only son. One day, a train was to cross the moveable bridge. Because of the operator's mistake, the bridge was lifted. His son saw the train and went up to pull the emergency lever, but slipped and got struck there. The man had still time to lower the bridge. But that would kill his son. If he didn't lower the bridge, it will kill hundreds of innocent people inside the train. So, how could he save both his son and hundreds of innocent people?
Source: 7 second riddles