Giant panda Tian Tian enjoys its birthday cake at the Smithsonian's National Zoo in Washington D.C., the United States, on Aug. 27, 2018. The zoo held a celebration on Monday for giant panda Tian Tian's 21st birthday. (Xinhua/Ting Shen)

Giant panda Tian Tian enjoys its birthday cake at the Smithsonian's National Zoo in Washington D.C., the United States, on Aug. 27, 2018. The zoo held a celebration on Monday for giant panda Tian Tian's 21st birthday. (Xinhua/Ting Shen)

Giant panda Tian Tian enjoys its birthday cake at the Smithsonian's National Zoo in Washington D.C., the United States, on Aug. 27, 2018. The zoo held a celebration on Monday for giant panda Tian Tian's 21st birthday. (Xinhua/Ting Shen)

Giant panda Tian Tian enjoys its birthday cake at the Smithsonian's National Zoo in Washington D.C., the United States, on Aug. 27, 2018. The zoo held a celebration on Monday for giant panda Tian Tian's 21st birthday. (Xinhua/Ting Shen)

People watch giant panda Tian Tian enjoying its birthday cake at the Smithsonian's National Zoo in Washington D.C., the United States, on Aug. 27, 2018. The zoo held a celebration on Monday for giant panda Tian Tian's 21st birthday. (Xinhua/Ting Shen)

Giant panda Tian Tian enjoys its birthday cake at the Smithsonian's National Zoo in Washington D.C., the United States, on Aug. 27, 2018. The zoo held a celebration on Monday for giant panda Tian Tian's 21st birthday. (Xinhua/Ting Shen)

Giant panda Tian Tian walks to its birthday cake at the Smithsonian's National Zoo in Washington D.C., the United States, on Aug. 27, 2018. The zoo held a celebration on Monday for giant panda Tian Tian's 21st birthday. (Xinhua/Ting Shen)

Zookeepers bring out the birthday cake for giant panda Tian Tian at the Smithsonian's National Zoo in Washington D.C., the United States, on Aug. 27, 2018. The zoo held a celebration on Monday for giant panda Tian Tian's 21st birthday. (Xinhua/Ting Shen)