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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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Don't you address how it could be different by suggesting that revealing text might be considering "writing", with is not a problem with sunglasses? –  Michoel Mar 10 '13 at 11:35
    
A similar question might be scrapping off "scratch cards" (like in some lotteries) on Shabbos to reveal the text underneath. –  Michoel Mar 10 '13 at 11:36
    
    
this site communitym.com/article.asp?article_id=101963 quotes R. Ovadia Yosef's explanation as to why it is OK. –  Danno Mar 10 '13 at 13:36
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Re. writing: If the writing is hidden beneath the thermochromic layer, how would this be different from opening a book to reveal the writing on the inside page? Or are you instead saying that the writing is embedded in the thermochromic layer and is exactly the same color such that, for all practical purposes, no writing exists before the color change? In other words, please specify the manner in which the writing is concealed. –  Fred Mar 10 '13 at 19:00
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1 Answer 1

In Shemirath Shabbath by Rav Yehoshua Y. Neuwirth, 40:2:

This chapter talks about medical procedures on shabbos.

The Halacha starts by saying

a. 1) It is permitted to measure body temperature on Shabbath.

Then continues:

f. 1) Whether or not one may use a forehead thermometer (consisting of a strip of celluloid which changes color, according to the temperature, when held in contact with the forehead) depend on its type.

    2) It may be used
        a) if the temperature is read from a change of color alone, and form of letter, digit or other symbol appears, or
        b) if the temperature is read by means of letters, digits or others symbols which are visible before use, but change color or become more pronounced when the thermometer is placed in contact with the forehead.

    3) If the letters, digits or other symbols by which the temperature is read are not visible before use, the thermometer may not be used.

There may be a leniency here because of a sick person, I don't know. But it's certainly clear that if the writing in the cup is not visible before heating, it's certainly prohibited.

Assuming there is no leniency (and I don't think one is being used since there are no qualifyers in a. 1, but of course I can not rule on this), then a change of color in the cup would be permitted, and making letters change color is also permitted, as long as they were visible before hand.

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