A novelty cup, see for example here, appears black when cold. When hot liquid is poured into it, the black changes to white and the previously concealed writing (underneath, so to speak) becomes visible.
Is it allowed to use this cup on Shabbos? You might think that the text has been "written" insofar as it could not be read before the hot liquid was placed in the cup.
If not, in what way is it different to photochromic prescription sunglasses which are allowed see "Halachically Speaking" page 8.
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