Breeders embrace four six-month-old giant panda cubs to do outdoor exercise at the Chongqing Zoo in southwest China's Chongqing on Dec. 22, 2019. The zoo held on Sunday a half-year-old birthday celebration for the four panda cubs named Shuangshuang, Chongchong and Xixi, Qingqing, which combine to symbolize "double joy and happiness" in Chinese. (Xinhua/Tang Yi)

Four six-month-old giant panda cubs play at the Chongqing Zoo in southwest China's Chongqing on Dec. 22, 2019. The zoo held on Sunday a half-year-old birthday celebration for the four panda cubs named Shuangshuang, Chongchong and Xixi, Qingqing, which combine to symbolize "double joy and happiness" in Chinese. (Xinhua/Tang Yi)

Four six-month-old giant panda cubs play at the Chongqing Zoo in southwest China's Chongqing on Dec. 22, 2019. The zoo held on Sunday a half-year-old birthday celebration for the four panda cubs named Shuangshuang, Chongchong and Xixi, Qingqing, which combine to symbolize "double joy and happiness" in Chinese. (Xinhua/Tang Yi)

Two six-month-old giant panda cubs play at the Chongqing Zoo in southwest China's Chongqing on Dec. 22, 2019. The zoo held on Sunday a half-year-old birthday celebration for the four panda cubs named Shuangshuang, Chongchong and Xixi, Qingqing, which combine to symbolize "double joy and happiness" in Chinese. (Xinhua/Tang Yi)

Four six-month-old giant panda cubs play at the Chongqing Zoo in southwest China's Chongqing on Dec. 22, 2019. The zoo held on Sunday a half-year-old birthday celebration for the four panda cubs named Shuangshuang, Chongchong and Xixi, Qingqing, which combine to symbolize "double joy and happiness" in Chinese. (Xinhua/Tang Yi)

Two six-month-old giant panda cubs play at the Chongqing Zoo in southwest China's Chongqing on Dec. 22, 2019. The zoo held on Sunday a half-year-old birthday celebration for the four panda cubs named Shuangshuang, Chongchong and Xixi, Qingqing, which combine to symbolize "double joy and happiness" in Chinese. (Xinhua/Tang Yi)

Two six-month-old giant panda cubs play at the Chongqing Zoo in southwest China's Chongqing on Dec. 22, 2019. The zoo held on Sunday a half-year-old birthday celebration for the four panda cubs named Shuangshuang, Chongchong and Xixi, Qingqing, which combine to symbolize "double joy and happiness" in Chinese. (Xinhua/Tang Yi)

Two six-month-old giant panda cubs play at the Chongqing Zoo in southwest China's Chongqing on Dec. 22, 2019. The zoo held on Sunday a half-year-old birthday celebration for the four panda cubs named Shuangshuang, Chongchong and Xixi, Qingqing, which combine to symbolize "double joy and happiness" in Chinese. (Xinhua/Tang Yi)