A crow (乌鸦) is thirsty. He finds a little water in a bottle (瓶子). But the neck (颈部) of the bottle is long. The crow can't get water. The crow thinks (想) and thinks. He puts some little stones in the bottle. The water rises up (上升). Now he can drink the water. What a clever crow!

简答：如图所示，斧子的手柄松了，用手柄的下端撞击树墩，斧子就被套紧了，请用物理知识解释这种现象．

 如图所示，开关闭合，两个灯泡都不发光，电流表指针几乎不动，而电压表指针有明显偏转，该电路故障可能是（　　）

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