If a story is set in medieval times is it ok to compare something then to something in modern times, in a description? For example; The decayed corpse was still bubbling like freshly melted cheese on toast. The sourcerer's magic box was the size of an i-phone. These are not the terms I want to use but just wondering if I have to keep the comparison medieval as well. Will it take the reader out of the story and say WTF?