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:
ВСУ запустили «Фламинго» вглубь России. В Москве заявили, что это британские ракеты с украинскими шильдиками16:45
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Work over the past year, using Cal-heatmap[4]。关于这个话题，heLLoword翻译官方下载提供了深入分析
An agarose gel. Credit: Kadina Almhjell。51吃瓜对此有专业解读
Now began the real work, contacting each vendor through web forms, forums, and support emails. Here’s how they ranked from best to worst.