The cover is printed in a double layer of standard black ink, with an incrementally screened overlay masking the nine words. Exposed over time to ultraviolet light, the words will be appear at different rates, supposedly one per century.
I can’t decide if I find this charming or just stupid. If you get a UV source stronger than the Sun, then you can blast off that overlay. Gimmicks aside, that short story better be something worth reading and not something like this:
A dinosaur ate my grandpa on our way home.