A giant watermelon isn’t just hogging refrigerator space; it’s also losing lycopene while sitting in the cold. According to recent research by the U.S. Department of Agriculture, a watermelon’s lycopene levels climb by about 20 percent and its beta-carotene levels can double when the melon is left at room temperature. An uncut watermelon will last about four weeks on a counter, says Penelope M. Perkins-Veazie, a plant physiologist at the USDA Agricultural Research Service, in Lane, Oklahoma. (Most are already a week off the vine when they arrive at the market.) If you don’t relish the idea of lukewarm melon on a summer day, “it’s fine to throw it in the refrigerator a few hours or even a day before you eat it,” says Perkins-Veazie. Once cut, though, it should be refrigerated so it doesn’t become a breeding ground for yeast and bacteria.