Usually "First events" play a very useful role and constitute a treasury used by the teacher. Such "first events" useful in teaching may be refraction of light on a prism, or observation of a flying balloon, or idea of Faraday's cage. On the other hand they can be an obstacle, eg. balloon filled with heliumcouse thinking that balloon, is always "lighter than air".
By "first concepts", first sensations, first way of comprehension we understand those that have left an imprint on memory, have become consciously well assimilated, and serve further on as points of reference for future mental events.
The barrier of "first mental events" manifests itself when new events cannot be simply added on to the old set in a conflict-free way.
Let give some examples of barriers that need to be conquered for instance mastering the division of larger numbers by smaller ones when learning how to divide smaller numbers by larger ones, understanding the concept of arithmetic means when learning about average velocity, or noticing that pressure increases with rising temperature when studying the ideal gas equation.
The obstacle of first understanding help recognize difficulties in understanding of quantum physics, noneuclidean geometry. For many students, understanding classical physics constitutes quite a barrier while studying relativistic and quantum mechanics [7].