Photo taken on July 23, 2019 shows a panda cub in an incubator at Chongqing Zoo, southwest China's Chongqing. Chongqing Zoo held an event Tuesday to celebrate two pairs of panda twins reaching one month old. Two giant pandas in the zoo in Chongqing gave birth to two pairs of twins on June 23. (Xinhua/Tang Yi)

Photo taken on July 23, 2019 shows a panda cub in an incubator at Chongqing Zoo, southwest China's Chongqing. Chongqing Zoo held an event Tuesday to celebrate two pairs of panda twins reaching one month old. Two giant pandas in the zoo in Chongqing gave birth to two pairs of twins on June 23. (Xinhua/Tang Yi)

Photo taken on July 23, 2019 shows a panda cub in an incubator at Chongqing Zoo, southwest China's Chongqing. Chongqing Zoo held an event Tuesday to celebrate two pairs of panda twins reaching one month old. Two giant pandas in the zoo in Chongqing gave birth to two pairs of twins on June 23. (Xinhua/Tang Yi)

Children view two pairs of panda twins in incubators at Chongqing Zoo, southwest China's Chongqing, July 23, 2019. Chongqing Zoo held an event Tuesday to celebrate two pairs of panda twins reaching one month old. Two giant pandas in the zoo in Chongqing gave birth to two pairs of twins on June 23. (Xinhua/Tang Yi)

Photo taken on July 23, 2019 shows a panda cub in an incubator at Chongqing Zoo, southwest China's Chongqing. Chongqing Zoo held an event Tuesday to celebrate two pairs of panda twins reaching one month old. Two giant pandas in the zoo in Chongqing gave birth to two pairs of twins on June 23. (Xinhua/Tang Yi)

Photo taken on July 23, 2019 shows a panda cub in an incubator at Chongqing Zoo, southwest China's Chongqing. Chongqing Zoo held an event Tuesday to celebrate two pairs of panda twins reaching one month old. Two giant pandas in the zoo in Chongqing gave birth to two pairs of twins on June 23. (Xinhua/Tang Yi)

A tourist takes a picture of a pair of panda cubs in an incubator at Chongqing Zoo, southwest China's Chongqing, July 23, 2019. Chongqing Zoo held an event Tuesday to celebrate two pairs of panda twins reaching one month old. Two giant pandas in the zoo in Chongqing gave birth to two pairs of twins on June 23. (Xinhua/Tang Yi)