Cause explains the reason how something has happened or occurred. It helps in finding answers to how an incidence took place. The question one should ask when one sees a cause clue word would be "How did it happen?"
In this way the cause can be answered with answers explaining how it happened. For instance, when Jim asked Tom "How did it happen?", Tom replied "It happened when I kept the candles close to the screen so that I can do a hand shadow puppet".
