Moldability criteria and process optimization for both reactive and thermoplastic injection molding hinge on the mold‐filling step. The fluid mechanics of the filling step is typically described in terms of a one‐dimensional main flow and a complex two‐dimensional flow near the advancing front, often termed the “fountain flow.” A unique apparatus which permits direct observation of the fountain flow in a rectangular cavity is described. The motion of tracer lines as well as the pathlines have been photographed for both Newtonian and shear‐thinning liquids. The data show clearly the main flow, the transition to the front flow, and the deceleration and acceleration zones in the fountain flow, which lead to a “mushrooming” of the tracer line. In addition, Galerkin finite‐element analysis is used to predict the isothermal free‐surface flow of a Newtonian liquid near the advancing front between parallel plates. The most interesting visualization of the calculated flow is obtained by tracking lines of material, thus producing numerical tracer experiments. The calculations show the widely varying residence times and the complex shear and extensional deformation histories of fluid elements as they move through the fountain flow region. The calculations are in agreement with the experiments and clarify previous results, both theoretical and experimental, that did not capture the details of the entire flow field.