五代蜀国后主孟昶一天晚上和花蕊夫人在宫中摩呵池上纳凉，写了两句词描述花蕊夫人容貌:“冰肌玉骨，自清凉无汗。”美丽的女郎看起来往往好像玉琢冰雕不染一尘，孟昶以为她们“自清凉无汗”，英国人以为butter wouldn't melt in their mouths(牛油在她们嘴里都不会融化)，原因是不难明白的。当然，butter wouldn't melt in her mouth和“自清凉无汗”一样，只是一个错觉。从前，这句英文成语大概有赞美含义，赞美少女天真无邪的样子；可是，成语意思渐渐改变，现在可以兼言男女，说他们“一副正经模样，但是--”那“但是”后面当然就是批评的话了，例如:He looks as if butter wouldn't melt in his mouth,but I'am convinced he is behind all this(他看来十分老实，但是，我深信这一切都是他挑起的)。