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We performed an experimental study in order to investigate the stabilityof crack propagation in a thin elastic sheet under an out-of-plane shear mode (modeIII). We find that a single propagating crack always follows a straight path, while twosimultaneously propagating cracks interact: their paths merge, forming a tongue-likeshape. Moreover, the experimental setup makes it possible to understand how energyintroduced at a large scale is focused at the crack tip. We find that, although theloading induces out-of-plane deformations, the material is locally broken in an openingmode (mode I).