In the framework of a curriculum reform in the Netherlands, an interdisciplinary open inquiry assignment has been introduced as a part of the final secondary school examinations. We developed and evaluated a preparatory open inquiry assignment to be used in science and mathematics departments of schools, in collaboration with each other: the falling cones assignment. However, teachers encountered two content problems, that hindered the (guiding of) the open inquiry process. Physics educators solved the problems and showed how the solution can be used in guiding students for inquiry learning.