A very thirsty crow(乌鸦) saw a water bottle and flew to it, hoping to find water inside.
He realized that there was too little water for him to reach to drink.
He tried everything he could to reach the water, but failed. At last, he collected as many stones as he could carry, and dropped them one by one into the bottle, until the water level was high enough for him to drink.
MORAL: Necessity is the mother of invention.