Thanks to: Peter Prestipino USA.

Three old ladies are sitting in a diner, chatting about various things. One lady says, "You know, I'm getting really forgetful. This morning, I was standing at the top of the stairs, and I couldn't remember whether I had just come up or was about to go down." The second lady says, "You think that's bad? The other day, I was sitting on the edge of my bed, and I couldn't remember whether I was going to bed or had just woken up!" The third lady smiles smugly. "Well, my memory's just as good as it's always been, knock on wood." She raps the table. With a startled look on her face, she asks, "Who's there?!"

三个年老的妇人坐在一起吃饭聊天.一个妇人说,"你知道,我现在是越来越健忘了.今天早上,我站在楼梯上,却想不起我是刚上来还是要下去了." 第二个妇人说,"你想这就严重了? 有一天,我坐在床边, 却想不起我是要去睡觉还是刚刚醒来!" 第三个妇人沾沾自喜地笑了一下,"好了,我的记忆还和原来一样,上帝保祜." 她敲敲桌子, 脸上出现一种惊愕的表情,她问,"谁在那里?!"

知识链接(百度):

　　Knock on wood 按字面的意思就是“敲敲木头”, 这个短语是指接触木制的东西可以确保好运、甩掉坏运气。这是一个典型的英语惯用语。例如：I have never been without a job, touch wood!

这个短语的起源和宗教信仰或迷信有些关联。它很可能起源于古代人的一种信念，即碰一碰树木可以驱邪或可以找到保护神，当时人们认为保护神是住在树里面的，特别是橡树。
　　也有的人认为knock on wood 起源于基督徒发誓或祈祷好运时触摸一个木制十字架的仪式。早期的基督徒认为他们所触摸的正是耶稣殉难的那个十字架。到了后来他们认为只要触摸任何木制十字架或是十字架的一部分，其意义都和触摸耶稣殉难十字架是一样的。
　　在英国有一个短语意思和knock on wood 一样，这个短语是touch wood 。它最早见于1908年的《威斯敏斯特公报》上。它有可能起源于19世纪英国一种叫做"touch-wood"的游戏。依照游戏规则，触摸到木头的人就可以不被捉到。
　　Knock on wood 在挪威有一个变体，即knock on the table 。既然在室内很难找到木头，找一件包含木制材料的家具代替木头也就顺理成章了。