In this paper we illustrate the use of qualitative reasoning models to assess the consistency of a hypothesis about the functioning of a black box system.To that purpose we have built a qualitativemodel of a mechanism for the periodic spring of Fontestorbes (Ariège, France) based on the siphon's hypothesis. The spring, called fountain locally,is a black box system because the mechanism is not accessible. Our model exhibits a cyclic behaviour that matches well with the periodic flow observed at Fontestorbes. It also generates the unexpected behaviour that was used to challenge the consistency of the hypothesis. The simulation of the expected behaviours appears to be a necessary condition to assert the consistency of the hypothesis while the generation of unexpected behaviours provide directions to challenge it. The mechanism at work characterizes a nonlinear relaxation oscillator,its modelling is our contribution to the modelling of natural phenomena.