Once during His visit to Chicago, USA, Swami Vivekananda was taking a stroll on the banks of a river. There he saw some young boys with an air gun trying to shoot at eggshells floating in the river. As the shells rolled up and down on the choppy water, the boys could not hit them. None of the boys hit an eggshell, despite trying many times.
Swami Vivekananda was watching their game with keen interest. Seeing this, the boys called out to Him, "Sir, you have been watching us all this time. Would you like to give it a try?" Swami Vivekananda smiled and said that He would like to try. Taking the air gun in His hand, He aimed at the eggshells and concentrated on the target for some time. Then He fired the gun twelve times and hit an eggshell each time!
The boys were awestruck on seeing Swamiji's skill and asked, "Well, Sir, how could you do this? Have you had a lot of shooting practice?" Swamiji laughed and said, "No, I have never practiced shooting." The boys were confused, "Then how is it that you shot so well?" Swami Vivekananda replied, "Well, let me tell you a secret. Whatever you do, concentrate all your attention on it. Think of nothing else. If you are shooting, keep your mind only on the target. Your aim will not fail. Concentration can work wonders. Even when you are studying, think only of the lesson at hand. Then you will easily remember what you study."
Moral: Concentration means not allowing the mind to think of anything except the target. Since the nature of the mind is such that it thinks continuously and gets easily distracted, it is difficult to achieve concentration. Daily chanting (repeating) of The Lord's Name is very helpful in achieving concentration.