A Minds on Learning article | Studycat’s series exploring the science of how children learn, for parents.

Most parents assume the clearer and more complete a learning resource is, the better. But when it comes to spelling and vocabulary, the opposite turns out to be true. Removing a few letters from a word — creating a small puzzle for your child to solve — produces measurably stronger recall than showing the full word every time. Researchers call this a desirable difficulty, and it’s one of the most reliable findings in cognitive psychology.

Key takeaways

• Seeing a complete word is easier but produces weaker memory. A small challenge — like a missing letter — makes the brain work harder and retain more.
• The effort of filling in a gap is what creates the memory. Passive recognition and active retrieval are not the same thing.
• This principle applies directly to language learning: children who actively recall vocabulary remember it significantly longer than those who simply see it repeated.
• You don’t need special tools to try this at home. A pencil and a piece of paper are enough.

What desirable difficulty actually means

When your child sees the word dog written out in full, their brain recognizes it — but recognition requires very little cognitive effort. The word is processed and moves on. No deep trace is left behind.

Now imagine the same word appears as d_g. The brain has to stop. It searches for what fits. It retrieves the missing letter from memory — and that retrieval process is where learning happens. Cognitive psychologist Robert Bjork, who has studied desirable difficulties for decades, found that adding just enough challenge to a task (without making it overwhelming) consistently improves long-term retention.

The key word is desirable. The difficulty has to be manageable. A child who can almost solve the puzzle learns from the effort. A child who is completely lost learns nothing useful. Missing one or two letters from a familiar word sits squarely in the productive zone.

Why it works especially well for spelling

Reading a word and spelling a word draw on different cognitive systems. A child can recognize zebra instantly but struggle to write it from memory. Passive exposure builds recognition. Active retrieval — including filling in missing letters — builds the kind of recall needed for real spelling.

Research by Duplice (2024) on second-language vocabulary learning found that generation tasks (where learners actively produce or complete a word rather than passively receive it) produced significantly better retention outcomes. The same principle holds for children learning to read and write in any language. When a child writes z_br_ and works out that it becomes zebra, they are doing more cognitive work than when they simply copy it. That extra work is the point.

The difference between recognizing and knowing

There’s a useful distinction worth keeping in mind: recognition and retrieval are not the same thing as knowing. A child who recognizes a word when they see it has only done part of the work. A child who can retrieve it, spell it, and use it in context has built something more durable.

Missing-letter activities bridge that gap. They move vocabulary from the “I’ve seen it before” category into the “I can actually use this” category — which is where real learning lives.

Try it at home:

Take any five words your child is working on — colors, animals, or food work well. Write each one with one or two letters removed and ask them to fill in the blanks. Then ask them to say the word out loud before moving on. That combination of retrieval and production gives the words the best possible chance of sticking.

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Scientific references & further reading

• Bjork, R. A., & Kroll, J. F. (2015). Desirable difficulties in vocabulary learning. UCLA Bjork Learning and Forgetting Lab.
• Duplice, J. (2024). Generation and L2 vocabulary learning: A classroom action study on the efficacy of generation — a desirable difficulty in learning L2 vocabulary. ERIC.
• Kühl, T., & Bertrams, A. (2019). How desirable are “desirable difficulties” for learning in educational contexts? Frontiers in Psychology.
• Metcalfe, J. (2017). Learning from errors. Annual Review of Psychology, 68, 465–489.
• McDaniel, M. A., & Butler, A. C. (2010). A contextual framework for understanding when difficulties are desirable. In A. S. Benjamin (Ed.), Successful remembering and successful forgetting (pp. 175–198). Psychology Press.

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