High challenge can mean more learning

An important piece of research comparing learning starting with easier problems, starting with harder problems or having them in random order. Probably our instinct would be that starting with easier problems would be the better way. However, as this article Using Challenging Concepts To Learn Promotes Understanding Of New Material tell us: "the effects of the different training methods depended on the type of categories that the participants were learning. When the categories could be easily described (i.e. was the line horizontal or vertical?), all three of the training procedures were equally effective. However, when the categories could not be described easily, starting with the harder problems then moving to easier ones produced the best results."
It would seem that the Adam naming the animals paradigm (where there is no preconceived system; you have to sort it out for yourself) is the most effective ...