What is that round thing in Ta 'er Temple? Is there any legend?

This is a prayer wheel! The prayer wheel is also called "Mani" prayer wheel (Sanskrit horse? 0? 3i, which means Ruyi Zhu Bao in Chinese, is related to the six-character mantra. Tibetan Buddhism believes that the more you read the six-character mantra, the more devout you are to Buddha and Bodhisattva, and you can get rid of the pain of reincarnation. Therefore, people not only recite the scriptures orally, but also make a "Mani" prayer tube, put the "six-character Daming Mantra" into the prayer tube and turn it by hand. Every time Tibetans put the scriptures into the prayer drum, it is equivalent to reciting the scriptures once, which means that the Six-character Daming Mantra has been recited hundreds of times. Some also use water power and light heat energy to make water-to-mani tubes and light-to-mani tubes, and natural forces can recite the Six-character Daming Mantra instead of people. Rows of prayer wheels are placed in front of large and small temples in Tibetan areas, with handles at the lower ends that can be pushed and shaken. Believers often go to temples to push the prayer wheel to rotate, which is called prayer wheel. The prayer wheel shaken by Tibetans is also called "Mani Wheel".