Though heterogeneous nucleation of liquid droplets on a smooth surface (such as a bathroom mirror) is a classical topic in nucleation theory, it is not well-known that this topic is actually a pedagogical example of heterogeneous catalysis: the one and only effect of the surface is to lower the activation Gibbs energy of droplet formation. In contrast to, for example, catalysis involving molecular reactions, the lowering of the activation barrier can be calculated with basic thermodynamics. This is an instructive exercise for a physical chemistry course that also explains to students why boiling stones have to be added to distillation vessels. (Contains 2 figures.)