Linguistic question - is 'Holy smokes'! a think you say when you get to the letter s of the other word, realize there are kids around, and scramble for a replacement, or is it an expression on it's own?
Nowadays, yes, but it's quite possible that both were earlier used to avoid "Holy Ghost" (= Святой_Дух, though the use of "Ghost" instead of "Spirit" is a bit of an archaism). I seem to vaguely remember hearing that it used to be used in the same context as "Jesus Christ" or "Oh my God" might be used nowadays as an expression of shock/anger/etc, and all three phrases are offensive in religious circles when used in such a way, as they're seen as violating the third commandment. Terms like "Gee whiz", "cripes", and "Oh my gosh" came into being as means of avoiding breaking the third commandment, and the phonetic structure of the word "smokes" in "Holy smokes" suggests that it was meant to be an avoidance of "Ghost". But the use of the term "Holy Ghost" in that context, if it ever was used, is further in the past, so I'm less sure of the origins of "Holy smokes" than I am of "Oh my gosh".
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As a further debunking of that myth, I've heard it mentioned that while jet fuel does not burn hotter than the melting point of steel, aluminum does, and there was plenty of aluminum in the structure of the aircraft involved, plus an ignition source provided by the jet fuel fires, and aluminum-water chemistry (with the water supplied by broken water mains in the buildings) would have made the aluminum fire even worse.