Giant panda twins Hehe and Jiujiu enjoy themselves in an air-conditioned room at the Nanjing Hongshan Forest Zoo in Nanjing, capital of east China's Jiangsu Province, Aug. 10, 2018. Staff workers at the zoo celebrated the 3rd birthday anniversary for the female giant panda twins "Hehe" and "Jiujiu" on Friday. (Xinhua/Su Yang)

A giant panda twin enjoys itself in an air-conditioned room at the Nanjing Hongshan Forest Zoo in Nanjing, capital of east China's Jiangsu Province, Aug. 10, 2018. Staff workers at the zoo celebrated the 3rd birthday anniversary for the female giant panda twins "Hehe" and "Jiujiu" on Friday. (Xinhua/Su Yang)

A giant panda twin eats birthday cake in an air-conditioned room at the Nanjing Hongshan Forest Zoo in Nanjing, capital of east China's Jiangsu Province, Aug. 10, 2018. Staff workers at the zoo celebrated the 3rd birthday anniversary for the female giant panda twins "Hehe" and "Jiujiu" on Friday. (Xinhua/Su Yang)

Giant panda twins Hehe and Jiujiu enjoy themselves in an air-conditioned room at the Nanjing Hongshan Forest Zoo in Nanjing, capital of east China's Jiangsu Province, Aug. 10, 2018. Staff workers at the zoo celebrated the 3rd birthday anniversary for the female giant panda twins "Hehe" and "Jiujiu" on Friday. (Xinhua/Su Yang)

A giant panda twin eats birthday cake in an air-conditioned room at the Nanjing Hongshan Forest Zoo in Nanjing, capital of east China's Jiangsu Province, Aug. 10, 2018. Staff workers at the zoo celebrated the 3rd birthday anniversary for the female giant panda twins "Hehe" and "Jiujiu" on Friday. (Xinhua/Su Yang)