这样一则民间故事：一对夫妇晚年得子，十分高兴，把儿子视为掌上明珠，捧在手上怕飞了，含在口里怕化了，儿子长大以后什么事都不让他干，连基本的生活也不能自理。一天，夫妇俩要出远门，怕儿子饿了，于是想了一个办法，烙了一张大饼，套在儿子的颈上，告诉他想吃时就咬一口。但是等他们回到家里，儿子还是饿死了。原来他只知道吃颈前面的饼，不知道把后面的饼转过来吃。
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    My uncle Jack is an interesting man. When he speaks, his mustache（胡子） always dances. It is so funny. He wears glasses and a funny hat.

    I don't like my boss（老板） at all. He is heavy with two small eyes. I think his glasses are too small for him.

— Look, the salesman（商人） is coming.

— Which one?

— The one with short black hair. He always takes his umbrella(雨伞) and suitcase(旅行箱). And his tie is very ugly.

— What are you doing, Lucy? Look at your pants. They are so dirty!

— I am doing some cleaning. Don't come here, Jane. Your beautiful skirt will be dirty.

某同学制氧气时，试管炸裂了，造成试管炸裂的原因是（　　）