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:
这种心理结构，为众多原生宠物品牌提供了明确的切入口。
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accounts, since they need to be able to reconcile where their cash went. And
Екатерина Смирная (корреспондент отдела оперативной информации)