That is my youngest's favorite line from Minimus Latin, chapter one. You see in chapter 1, the mouse, Minimus, goes around to the people at Vindolanda asking who they are, and they answer in turn. At last, Minimus makes it to the cat, and asks "quis es" in a tiny, shakey little voice. The cat answers, and the final scene has Minimus wasting no time getting away, with the words "mus exit". Little Luke Skywalker thinks it is hysterical, and whenever we practice asking "quis es?", he will answer "mus exit".
Tonight we were sitting on the couch; the older two were taking turns reading to me from their HOP workbooks. The youngest was sitting there with us. Suddenly I heard his little voice say, "mus exit." I look over and he is "reading" through the Minimus book - every time he turned the page to a new dialogue, he would say, "mus exit." I'm glad to see he is enjoying Latin so much. :) If he's not going around saying "mus exit", then he can usually be found singing the Vale song from Song School Latin. Who knew that the Latin programs I bought for the older two would enchant the youngest so much.