The JuxtaLearn project(funded by the EU)aims at facilitating the acquisition of science concepts through the creation and sharing of videos on the part of the learners.For the specific learning targets threshold concepts are specified as key elements of knowledge.Content analysis techniques are used to extract learners' concepts manifested in textual artifacts taken from online resources like Khan Academy and YouTube comments.Deviations between student concepts and an agreed upon domain knowledge represented in an ontology can indicate problems of understanding.In this paper we particularly report on the judgement of teachers reading the validity of the problems of understanding identified by our method.