Fred wrote: << Which reminds me --- quite irrelevantly --- of a claim I fear I may have made previously in these august pages: that a most effective means of causing a polite gathering to descend into an incoherent brawl is to remark that "a mirror exchanges left with right" and then to enquire innocently "so, why doesn't it also exchange up with down?"

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No problem -- you just rotate the mirror 90 degrees, and voilà! --Dan