Mixing Problem Types Feels Worse and Works Better

I remember sitting in a windowless training room five years ago, staring at a glossy PowerPoint presentation that promised to revolutionise our department. The presenter was droning on about the cognitive science behind “interleaving,” painting this picture of elegant, seamless transitions between complex topics that would somehow unlock genius in every student. It sounded lovely in a vacuum, but as I sat there, I couldn’t help but wonder how much this theory actually accounted for the reality of a Year 9 class on a rainy Tuesday afternoon. We talk about how interleaving affects learning as if it’s a magic switch we can just flip, but in a real classroom, trying to mix up topics mid-stream often feels less like “cognitive flexibility” and more like total instructional chaos.

I’m not here to sell you on another pedagogical fad or a way to make your lesson plans look prettier for an observer. What I want to do is strip away the academic jargon and look at what actually happens when you try to implement this in a room of thirty kids who are already struggling to find their pens. I’m going to tell you how to use it without losing the thread of your lesson, because if a strategy doesn’t survive the contact with a chaotic Period 5, it isn’t worth your precious time.

Why Blocked Practice vs Interleaved Practice Feels Like Failure

The reason most of us recoil when we first try interleaving is that, in the moment, it feels like we’re failing. When you use blocked practice—just hammering one specific skill until the whole class seems to “get it”—the lesson feels smooth. There’s a rhythm to it. The kids are getting the answers right, the momentum is building, and you feel like you’re actually winning. But that’s the trap. It’s a false sense of mastery.

When you switch to blocked practice vs interleaved practice, the latter feels messy. Because you’re forcing students to jump between different types of problems, they lose that easy momentum. They start stumbling. You might see hands go up with more uncertainty, or worse, a heavy silence. This is where desirable difficulty in education comes into play; it’s supposed to feel hard, but when you’re standing there in a room of thirty teenagers, it just feels like they aren’t learning. You start questioning if you’ve lost the plot, when in reality, you’re just finally asking their brains to do the heavy lifting.

Embracing Desirable Difficulty in Education Without Losing Your Mind

The problem with the term “desirable difficulty in education” is that it sounds far more pleasant than it actually feels in a classroom. In theory, it’s about making the brain work harder to ensure deeper encoding. In practice, it often feels like you’re watching your students struggle, and that triggers a very specific kind of teacher guilt. You see them frowning at a problem that mixes three different concepts, and your instinct is to jump in and rescue them with a simplified, “blocked” version of the task.

But here’s the thing: if they aren’t struggling slightly, they aren’t actually learning; they’re just performing. When we lean into pattern recognition and discrimination learning, we are essentially teaching them how to choose the right tool for the right job, rather than just following a recipe. It’s the difference between a student who can solve a math problem because they’ve done ten identical ones, and a student who can actually see the problem for what it is. It’s exhausting to facilitate, and it’s even more exhausting to watch, but it’s the only way to move beyond the superficial.

Five Ways to Make Interleaving Actually Work Without Breaking Your Lessons

• Don’t try to interleave everything at once. If you’re trying to mix three different historical eras and five different skill sets in a single forty-minute period, you aren’t “promoting cognitive flexibility”—you’re just creating a fog of confusion. Start by mixing just two related concepts. Let them settle before you throw a third into the mix.
• Accept that the “dip” is real. When you move from blocked practice to interleaving, your students’ scores on immediate retrieval tasks will almost certainly drop. They’ll tell you they don’t “get it” and you’ll feel like you’re failing them. Don’t panic. That struggle is the sound of the brain actually doing the heavy lifting. If it feels easy, they probably aren’t learning anything that will stick past Friday.
• Use your low-stakes retrieval to bridge the gaps. Interleaving only works if they can actually access the previous bits of information. If you’re jumping between Topic A and Topic B, make sure your starter or your quick-fire questions act as the “glue” that reminds them why they are looking at something different today. You can’t expect them to leap across a chasm without a bit of scaffolding.
• Stop over-complicating your lesson planning. You don’t need a bespoke, colour-coded interleaved curriculum. You just need to stop treating every new topic like a sealed box. When you start a new unit, spend five minutes bringing back a concept from three weeks ago. It’s not a “new initiative”; it’s just not letting the old stuff rot in the back of their minds.
• Watch the room, not the theory. If you’ve attempted an interleaved task and the entire class has gone silent in that “I’m lost and about to start throwing pens” kind of way, abandon ship. There is a fine line between “desirable difficulty” and “unproductive frustration.” If they can’t find the thread, go back to the blocked practice for a moment. You can’t teach a student who has mentally checked out because the cognitive load just became a cognitive landslide.

The Reality Check: What Actually Sticks

Stop chasing the “perfect” interleave; if you try to jump between three different topics in a forty-minute lesson while the Year 9s are restless, you aren’t teaching—you’re just managing chaos. Aim for small, predictable shifts rather than total topical whiplash.

Accept that “productive struggle” looks like confusion, and that’s okay. If the students look like they’ve suddenly lost the plot when you introduce a new variable, it doesn’t mean your lesson has failed; it means their brains are actually doing the heavy lifting required to make the learning stick.

Prioritise the “why” over the “how.” Interleaving only works if the students can see the connection between the old task and the new one; if you’re just throwing random questions at them to tick a box for a CPD requirement, you’re wasting their time and yours.

The Illusion of Mastery

“The problem with blocked practice is that it feels good—it feels like progress because the kids are getting the answers right in the moment. But interleaving is different; it feels messy, it feels like they’re struggling, and frankly, it feels like you’re losing control of the lesson. Just remember: if they aren’t stumbling slightly, they aren’t actually learning to navigate the gaps between what they know.”

Ellery Mwangi-Selkirk

The Reality Check

So, where does that leave us? We’ve established that interleaving isn’t some magic wand, and it certainly isn’t a way to make your life easier on a Tuesday afternoon. It’s actually harder—for you to plan and, more importantly, for them to process. But the science holds up: that friction, that sense of struggle when a student has to pivot from one concept to a seemingly unrelated one, is exactly where the long-term memory gets its grip. You don’t need to overhaul your entire scheme of work by Monday morning. Just stop treating every lesson like a closed loop. Mix in a few questions from last month, break up the repetitive drills, and accept that learning should feel a bit messy sometimes.

If you’re feeling overwhelmed by the prospect of changing your rhythm, just remember why we bother with the theory in the first place. We aren’t trying to win awards for pedagogical perfection; we’re trying to make sure that when these kids sit their exams—or better yet, when they face a real-world problem three years from now—the knowledge actually stays with them. It’s easy to get lost in the “shiny” strategies, but the real wins happen in those small, difficult moments of retrieval. Keep it simple, keep it practical, and trust the process even when the classroom feels like it’s leaning towards chaos. You’re doing the hard work, and that’s what matters.

About Ellery Mwangi-Selkirk

Most educational advice is written for a classroom that does not exist — twenty motivated pupils, unlimited preparation time, no behaviour to manage. I write for the room you are actually in. What retrieval practice looks like when marking is already three weeks behind, why the seating plan matters more than the starter activity, and which of the last decade's initiatives left anything behind. Nineteen years taught me that the good ideas survive contact with a wet Wednesday and the rest do not.