Apathy Jack writes:
As every practising teacher understands, resistance is a necessary factor in teaching. Soft non-resistant material takes an imprint very readily only to lose it again very quickly. Easy pupils make teaching slipshod. The difficulty but also the soundness of teaching increases with the amount of reaction in the learner. And also resistance involves a certain amount of collaboration; the thing learnt becomes a resultant, incorporating elements introduced in the struggle. It is easier to carve cheese than a good piece of wood; every piece of wood has a bias, it has to be delt with on its own terms, it has to be managed and humoured, but in the end there is no comparison in the quality and interest between carved cheese and woodcarving.
-HG Wells, The Shape of Things to Come