"It takes advantage of a physiologic process to improve your exercise capacity and your recovery, " Ann said.

WSJ: One Kid's Personal Tennis Academy

And just as in micro-cap stocks, the most successful investors will be those who buy in for pure enjoyment, find a way to exercise some personal control over pricing or obtain some information advantage.

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In The Happiness Advantage, I challenge readers to do one brief positive exercise every day for 21 days.

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