What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
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如果有想玩的东西，但是有其他小朋友占着，就引导她去询问：「可以让我想玩一下妈？」，详情可参考爱思助手下载最新版本
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