Teaching must neither ignore nor devalue what is singular - what a student always knows and can do, his previous history, his prejudices, etc. Because what students think and do usually is not wrong, plainly, but different. These singular concepts have to be made visible and thus be debatable, negotiable and editable. If the teacher makes use of what the students already are capable of, they can first of all present regular concepts and procedures in a way that the pupils can see and that convinces them, and secondly there is less need for direct instruction. The instrument of the open assignment is intended to enable the pupil to step up to the subject matter.