In what ways could teachers' attempts to make mathematics understandable and easy to learn affect how the topic to be learned is handled in the classroom? From the point of departure of two Learning studies - a collaborative work among teachers aiming at exploring and developing what is made possible to learn in the classroom - I describe how the teachers' good will to facilitate learning by reducing complexity had an un-expected effect. It is also described how the teachers successively got to know that aspects of the object of learning, either taken for granted or deliberately avoided by the teachers, were necessary for learning. Two studies - teaching and learning to convert sentences into algebraic expressions and addition and substraction with negative numbers respectively - are reported.