Now and then, I’ve taught whet seemed to be a successful lesson. I’d explain challenging content, check for understanding, get some great responses to consolidation activities and, at the end of the lesson, students would troop out happy, confident and certain they’d grasped what ever it was I’d taught only for them to have seemingly forgotten it all by next lesson. Sound familiar? How is it that children can appear to have understood one day but forget the next?

In order to remember something, first you have to think about it. We can’t think about everything in the environment because we have a limited capacity to pay attention, but anything we think about is stored in long-term memory. Once something we’ve thought about is transferred to long-term memory it becomes knowledge. The human capacity to store information in long-term memory is unimaginably vast: there’s more than enough space to store everything we’ll experience in our long lives. So, why can’t we remember everything we’ve thought about?

The process of ‘storing’ information as knowledge is invisible to us. We have no ability to directly access long-term memory. Consciousness takes place in our working memories and we only have access to those memories we retrieve at any given moment in time. Memory is also context dependent. Cues and in the environment – Proust’s smell of madeleines is a famous example – prompt unbidden recollection. But the point remains: we have no direct ability to introspect about the contents of our long-term memory, how knowledge is stored or organised or how some things are recalled to working memory. But without these cues it can be hard to recall what we want to think about.

Remember the scene at the end of Raiders of the Lost Ark where the Ark of the Covenant is nailed into a wooden crate labelled Top Secret and stored in a vast warehouse full of identical crates?

This helps explain why we can’t always find what we’re looking for. If we don’t know very much about a subject it can be very hard to remember the few things we do know, but the more we know the easier it seems to become. When we travel abroad and try to learn a few words in a foreign language it’s tricky to remember them, whereas remembering the same words in our native tongue is much easier. The speculation is that we store information in long-term memory, it’s organised into inter-connected web of facts, ideas, examples, impressions and experiences that psychologists refer to as schemas.

If you learn one single word in a new language it will be unconnected to much else. This means there are few potential cues to help you recall it. However, when we learn lots of words and phrases, all the individual items become connected together and, when we recall one piece of knowledge, the rest of the schema is dragged along making it easier for us to think about the new knowledge we’ve learned.

When I think back to those seemingly successful lessons in which students seemed to have grasped a tricky concept, in reality, all that had happened is that they’d become familiar with the subject matter. There are two types of long-term memories: episodic and semantic memories. Episodic memories are those of experiences and specific events. Memories of how we felt are episodic and hep us to reconstruct events from our past in vivid detail. Semantic memories are a more structured record of facts, concepts and meanings. Semantic memories are stored independently to the specific context in which they were first learned and so can be more generally applied across a range of contexts.

Students memories of lessons are often episodic – they remember the experience, but forget the specific knowledge which a teacher might have been trying to embed. If you ask, D’you remember when we studied the Vikings? students will feel a sense of familiarity and say, Oh yes, I remember that. But then, if you ask what information they can specifically recall about the Vikings, they may struggle to remember anything useful. Strong semantic memories are formed when we have to retrieve factual information. This is why retrieval practice is so effective at helping students learn content.

If instead of practising retrieval we simply re-encounter information we’ve previously studied we get a familiarity effect: Oh yes, I remember that. But this familiarity leads to an illusion of knowledge. We think we know things we can’t actually recall. But if we’ve practised trying to retrieve information we’ll have a much more accurate picture of the extent of our ignorance. Knowing that we don’t know is far more useful to us that falsely believing that we do.

If we want students to move beyond mere familiarity to being able to recall the academic content of lessons, we need to consciously build semantic memories by making use of the spacing effect and reiterating important knowledge and by making use of retrieval practice to help strengthen academic schemas.