Tourists Joseph Lin, Ning Chao, center, and Linda Wang, left, pose for a selfie along the south rim at Grand Canyon National Park, Ariz. The throngs of tourists have been showing up in big numbers at other national parks, including Yellowstone in Wyoming, Yosemite in California and Zion in Utah, driven by good weather, cheap gas and marketing campaigns ahead of next year's National Park Service centennial. (Emery Cowan/Arizona Daily Sun via AP)