Practicing one skill over and over (blocking) feels productive but fades fast. Mixing different skills or problem types in one session (interleaving) feels harder and clumsier — and produces far stronger, more flexible learning. The struggle is the point.
Interleaving: instead of practicing one skill or topic repeatedly before moving on (blocking), mix several together in a session — it feels harder and less smooth, but builds deeper, more durable, more transferable learning.
The natural way to practice is blocking: do twenty of the same problem, master it, move on. It feels great — each rep gets smoother. But research in Make It Stick shows that smoothness is a trap. Students who interleave — mixing problem types so they never do two of the same in a row — perform worse during practice and far better on later tests. In one study, math students who interleaved doubled the long-term performance of those who blocked, despite finding it harder in the moment.
Why does the harder method win? Blocking lets you run on autopilot: once you know 'this is a type-A problem,' you stop thinking and just apply the procedure. Interleaving forces you, every single time, to first figure out which kind of problem this is and which approach it needs — which is exactly the skill you'll need in the real world, where problems don't arrive pre-labeled. The extra effort of switching is 'desirable difficulty': it feels like worse learning and is actually better learning.
To use it, resist the urge to drill one thing to fluency before the next. Mix related skills in a session — different chord types, different problem categories, different vocabulary sets — and tolerate feeling clumsier than blocking would. (It pairs with spaced repetition: interleaving naturally spaces each item out.) The discomfort is the signal it's working. If practice feels smooth and easy, you're probably building fluency that won't survive outside the practice room.
The practice that feels most productive (drilling one thing smoothly) builds the most fragile learning; interleaving feels worse and works better — so knowing this stops you optimizing for the comfortable illusion of progress.
It's not 'never repeat anything' or 'jumble everything randomly.' Beginners often need some blocking to grasp a method first, and interleaving works best among related skills. The point is that smooth, blocked practice overstates your learning — once you have the basics, mixing builds the durable, transferable version.
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