Hard fun
Papert's observation that people love work that is hard — when the hardness is in service of something they care about.
"Hard fun" is a phrase Papert reported hearing from a child, and embraced. The conventional wisdom says learning should be made easy and fun. Papert's correction: people — children included — enjoy difficulty when it is the right difficulty, in pursuit of a goal that is theirs.
Programming is the canonical example. Debugging is frustrating, but a child will happily fight a Logo program for an hour to make the turtle draw the house they imagined. The hardness isn't a bug in the experience; it's the source of the satisfaction.
The idea anticipates later research on flow and on productive struggle, and it quietly indicts a lot of "engaging" educational software that sands away exactly the difficulty where the learning lives.