"In a recent series of experiments, it has been shown that polar molecules can be decelerated, bunched, cooled, and trapped using time-varying electric fields. These experiments demonstrate an unprecedented level of control over molecules, which enables a variety of applications of great scientific interest. Here, I propose to use these techniques to create a molecular fountain. In this fountain, the first of its kind, polar molecules are decelerated, cooled, and subsequently launched upwards some 10-50 cm before falling back under gravity, thereby passing a microwave cavity or laser beam twice – as they fly up and as they fall back down. The effective interrogation time in such a Ramsey type measurement scheme includes the entire flight time between the two traversals through the driving field, which can be up to a second. This long interrogation time will allow for extreme precision measurements on molecular structure to a level at which fundamental physics theories can be tested. I will use the inversion frequency in ammonia around 23 GHz as a test case. This transition is very well studied and was used in the first ‘atomic’ clock and the first demonstration of a MASER. The fountain should make it possible to measure the inversion frequency with a relative accuracy of 10^{-12}–10^{-14}; that is more than a thousand fold improvement as compared to the best previous measurement. Besides serving as a proof-of-principle, this measurement may be used as a test of the time-variation of fundamental constants – an issue that has profound implications on how we understand the universe. The inversion frequency in ammonia is determined by the tunneling rate of the protons through the barrier between the two equivalent configurations of the molecule, and is exponentially dependent on the proton mass. By monitoring the inversion frequency over a period of a few years, a possible variation of the proton-electron mass ratio can be constrained or measured."