One fundamental problem of the constructivist misunderstanding of knowing and learning phenomena comes from the fact that those who know do not understand the non-knower's world and experience sui generis, as a phenomenon in its own right. It turns out, however, that the child's rationality is different because his/her access to and experience of the world is different; s/he lives in a different world (or rather, to be more precise, a different lifeworld). I provide a phenomenological analysis of "discovering" entities that had never seen before but in the same instance involved a forgetting that an immediately previous worlds was not populated with the entities. I assist readers in becoming aware of the processes that do not normally allow them to understand the states of knowledge of learners, and therefore to misinterpret their classroom observations and recordings thereof. This gives us a first understanding how constructivism works only when one already assumes (has attained) the later states of knowledge.