Snipped this from hoagie's face book.

Recognize how much effort is required to focus attention for long periods of time, persevere at challenging tasks, and stifle the desire to socialize with all of the people sitting around you. Children and adults alike do not have the brain power to sit and learn information for 8 hours per day (much less when the starting point is around 8am). Rather than viewing self-control as a mindset, a personality trait, a virtue, or a skill, consider it to be a renewable resource. Using self-control to direct our attention, delay gratification, resist impulses, or control our emotions, exhausts resources in the body. We have a limited amount of resources that can be depleted. No matter how physically fit we are, exerting ourselves inevitably leads to some degree of exhaustion. Acts of self-control deplete mental strength in the same way that exercise temporarily depletes physical strength. But there is one important lesson from science: people can replenish their mental strength and with training, people can increase their self-control capacity (such that it will take longer to reach the point of exhaustion).

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Youth lives by personality, age lives by calculation. -- Aristotle on a calendar