"I don't think you're as capable of handling lack of sleep or whatever challenges you throw at your body as you get older. However, I never missed a day of work."

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Discipline makes hard men. Every hard man is capable of being a killer. But every killer is not capable of being a hard man. They can't endure that much. It's all about the endurance. That's why they become killers. Because they can't endure the pain. They need to kill the pain to stop it.

— Mike Tyson