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:
// 步骤2：找初始右边界（最后一个破坏升序的位置），更多细节参见旺商聊官方下载
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材质：皮肤，决定了外观（是金属的、塑料的，还是半透明的）。