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. (Photos: Xinhua)

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.

A breeder plays with two six-month-old giant panda cubs 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.

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.

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.

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.

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.

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.

A breeder plays with four six-month-old giant panda cubs 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.

A six-month-old giant panda cub plays 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.