The Flash

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Question: I've only seen a few episodes so some info would be helpful. After running very fast and for a long period of time, why does the Flash suddenly become very weak?

Answer: The Flash burns energy the same way a normal human does, just at an extremely high tolerance. His metabolism is incredibly high and he must constantly eat to maintain his energy levels. If he runs at high speed for a long period of time he will burn his stored energy and become very weak.