My two-year-old daughter is a genius--a genius at doing things that drive me nuts, that is. Just last week, she was in the dog food bin, feeding herself handfuls of kibble for at least the millionth time.

"Lucy!" I said. "You know that makes Mama mad!"

"Mama," Lucy replied, barely pausing to swallow. "Be happy."

This made me stop and think. How is it that a two-year-old already knows about happiness? Where does she get the idea that happiness is something you can control? Most important, how can anyone be happy with dog food slurry dribbling down her chin?

So many times I've wondered about the secret to happiness. Why are some people naturally so cheery, and other people--like me--so prone to seeing the dog food sludge and not the happy smile?

Happiness has been a mystery for eons. Philosophers and religious thinkers have dissected it (much to the torment of freshman philosophy students). What's more, novelists have written books disparaging happiness. One of my favorites, Brave New World, describes a society where happiness and comfort are engineered. One implication is that the pursuit of happiness is a path to corruption.

But what modern scientists are learning is that happiness isn't such a bad thing. Psychology professor Sonja Lyubomirsky, who researches happiness at the University of California, Riverside, says happy people are more productive and make more money. They have more friends and their marriages last longer. They feel healthier. They're more creative, helpful, generous, and disciplined.

What's more, happiness helps you solve problems. In one study, 75 percent of the people who'd watched a funny movie were able to solve a problem, as opposed to 13 percent of the people who hadn't seen the funny movie.

Another scholar, Rotterdam-based professor Ruut Veenhoven, goes one step further.

"Happy people live longer," he says. Veenhoven should know; he manages the World Database of Happiness, which compares happiness levels in societies around the globe, among other things.

Here in the United States, we take happiness seriously. Our Founding Fathers even included it in the Declaration of Independence. We have a right to pursue it, along with life and liberty. But while the meanings of the first two things are pretty obvious, happiness is harder to define. Without instructions from the Founding Fathers, we've been left to figure it out ourselves.

Lucy seems to have come up with her own answer. Happiness is a forbidden bucket of dog food. That doesn't really do it for me, though. To find my answer, I decided to start with the classics.