We consider the problem of a tensile crack approaching a sliding interface in the direction normal to it in an attempt to find the mechanisms that control the crack offset. The crack in a plane free of loading is driven by uniform load applied to its faces. The interface is assumed to have no resistance to opening. The common conception is that as the crack approaches the interface it creates a zone of opening. When the crack touches the interface this opening zone eventually arrests the crack such that the continuation of the crack growth through the interface is only possible from an offset position. Our computer simulations conducted for frictionless interface and supported by a simple analytical model show that the situation is more complex. As the crack tip gets close, the zone of opening shrinks and the opening displacement increases. After the crack tip touches the interface, the opening zone disappears. Frictionless interface produces concentration of this stress only at the ends of the interface which physically corresponds to the points where sliding is artificially arrested.