Since most students "hate" the concept of limit, in order to make them "happier," this article suggests a couple of naive "lim (h[right arrow]0)-is-missing" problems for them to try for fun. Indeed, differential functional equations that are related to difference quotients in calculus are studied in this paper. In particular, two interesting observations are made in this article, namely, (1) it is possible to solve a differential functional equation just by some basic algebra; and (2) a certain class of smooth functions is characterized by imposing a simple condition on the value c, where c is guaranteed by the Mean Value Theorem for any smooth functions defined on any interval [a, b].