Let’s say there is a person walking along carrying some stones. He sees a dog, and throws his stones at the dog. He also sees a lion and he throws his stones at the lion, too.
What does the dog do? At the sight of the stone, the dog immediately tries to bite or chase it. Then the person gets to throw another stone at the dog. The dog again tries to follow the new stone and bite it. Now this guy has got a big collection of stones, so they are not going to run out any time soon. The dog becomes very tired.
Now when the man throws a stone at the lion, the lion does not look at the stone. Rather, he thinks, “Where did that stone come from? Who threw that stone?” When he sees the person who threw it, he pounces on him. A person only gets to throw one stone at a lion.
– Unknown origin. Heard from Mingyur Rinpoche, Tibetan Buddhism.