An AI text watermark leaves detectable evidence in the words a model chooses. During generation, a secret scoring rule helps select the next piece of text. Later, a detector with the matching key can reconstruct that rule and check whether the finished passage agrees with it unusually often.
The surprising part is that an exact copy into a plain-text document can preserve the evidence. There is no hidden character to carry along. The ordinary words preserve the choices.
If you use AI to draft, proofread, or rewrite, the distinction matters: those operations leave different amounts of freedom for a watermark. And when someone says a passage was “detected,” the test matters too. A keyed watermark check measures a deliberately introduced pattern; it does not infer AI use from a writer's style.
