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Who knew this juicy fruit could lift midday fog? Watermelon contains citrulline, an amino acid that helps open up blood vessels and boost bloodflow. "The better the bloodflow to your brain, the better your brain cells function and the more clear-headed you think," says Elizabeth Somer, RD, author of Eat Your Way to Happiness. It helps you gain perspective on your problems and make daily to-dos seem doable. What's more, watermelon is, well, water-rich. "Eating a big slice is like drinking water," says Somer. That's a great thing because staying hydrated is vital to maintaining a happy mood, say researchers. And finally, watermelon is a major source of the antioxidant lycopene, which helps reduce inflammation, a major player in mood disorders.