Volume 1, Home Education, pg 24
But anyone knows that, if you work too hard at some mental task, your brain becomes tired. If a child does very challenging math, his mind will get fatigued and he will start to have trouble and make silly mistakes. But if you switch activities and let him read some history, his mind is fine for that task. Using his imagination to picture history apparently uses a different part of the brain than doing math and, since it was dormant during the math, it is well-rested and ready to work for history. Schools often schedule lessons to mix up types of brain activities during the day, but parents often don't know that it's important to do this.