为什么他们在句子结尾说一句“敲木头”

表达：knock on wood 祈求好运，老天保佑

英语释意：“Knock on wood" is said when you hope something will happen without any problems. 

e.g. I've never had a single problem with it - knock on wood! 我车从来都没有过任何问题～老天保佑以后也不出问题啊！

e.g. If we work all night, we should be able to finish in time. As long as we don't fall asleep — knock on wood. 如果我们今天晚上弄一晚上，我们应该可以赶得上弄完。只要咱别睡着，老天保佑啊！