Learning is a change in long-term memory. If a pupil cannot recall and use information later, they have not learned it — they have merely encountered it. This single idea, drawn from cognitive science and embedded in the ITT Core Content Framework, should shape every lesson you plan.
Working memory is the bottleneck. It can hold only around four chunks of new information at once, and it forgets them within seconds unless they are rehearsed or connected to existing knowledge. When pupils look 'lost', they are usually overloaded, not unmotivated. Reducing extraneous load — fancy slides, off-topic asides, ambiguous instructions — frees capacity for the learning itself.