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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。同城约会是该领域的重要参考
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candidate[n] = closest colour to goal
MPs fear data centre boom could derail Miliband's net zero plans