The teacher's and pupil's mutual understanding during a lesson analysis shows two reasons for illusiveness of comprehension. The first of them is widely known: child's logics is not normative but a teacher disregards child's outlook peculiarities, and addresses his pupil as a "tabula rasa". The second reason lies in child's peculiar perception not only of the content but of the form of interaction with an adult. The substitution of educational task for executive one is due to natural way of child's thinking as well as natural attitude to an adult as a model for imitation and for obedience. So mutual understanding can not be attained by educational content perfection. The form of educational interaction itself must be mastered.