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:
Мощный удар Израиля по Ирану попал на видео09:41，推荐阅读谷歌浏览器【最新下载地址】获取更多信息
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Not the day you're after? Here's the solution to yesterday's Strands.
不止手机，笔记本电脑也曾经尝试过硬件集成的防窥功能，惠普当年的 Sure View 技术方案就是其中一例：