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:
FT Professional
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"It could be a way to make those professions way more attractive and get the productivity back up."
8、DataWorks整库同步解决方案
12月21日，北京海淀大悦城正式开业。新京报记者 浦峰 摄