A love potion. (Now we’re getting into the heart of the plot.) At this point Gilbert drags in a laborious pun on the word filter: an earthenware jar for purifying drinking water. When the opera was written (1877), theater managers were in the habit of leaving the house lights up. Audiences could buy copies of the librettos, which they were fond of reading word-for-word throughout the show. Being thus informed, they could tell the difference between philtre and filter. Modern audiences cannot, and directors could do us a favor by omitting that particular exchange altogether.