We need a level of stress to engage our brain. And this is provided by having a task that is difficult. But not too difficult. and not too easy
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If the difficulty level is too hard, the brain will feel it is suffering and just stop
If too low, then the brain will not engage
When the level is just right, cortisol (stress hormone) is released and we get a sense of satifaction.
When we are learning something, the brian is engaged.
Just repeating something over and over again doesnt produce results. We need to make intentional practise - focus on a particular area and consciously try to improve it.