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Malk (Pronunciation: /mʌɫk/; rhymes with "balk") is a drink which is served with the lunches at Springfield Elementary School.
Bart Simpson thought that the school served milk with the lunches, until he took a closer look at a carton and saw that it was labelled "MALK" rather than "MILK".
Malk's specific nutritional qualities aren't known except that it includes Vitamin R, according to the verbiage on the carton. It is also either completely lacking in calcium (and other bone-strengthening nutrients) or has another, unspecified, substance that actively weakens bones. Bart cracked his knuckles, heard a snap and complained about his bones being brittle, in spite of the fact that he always drank plenty of Malk.