Because the glutinous rice balls are placed in the water in the pot, the gravity they receive is greater than the buoyancy, so they sink; the cooked glutinous rice balls expand due to the internal heat expansion, the volume becomes larger, and the buoyancy force they experience becomes greater. When the buoyancy force is Greater than gravity, the glutinous rice balls float upward.

Tangyuan, also known as "Yuanxiao", "Tangtuan" and "Floating Yuanzi", is one of the representatives of traditional Chinese snacks. It is a spherical food made of glutinous rice flour. Usually there are fillings, cooked and served with soup. It is also the most distinctive food of the Lantern Festival and has a long history.

It is said that glutinous rice balls originated in the Song Dynasty. At that time, Mingzhou (now Ningbo City, Zhejiang Province) started to eat a novel food, which was filled with black sesame and lard, a little white sugar was added, and the outside was rolled into a round shape with glutinous rice flour. After cooking, it tasted sweet and delicious. Very interesting. Because this kind of glutinous rice dumpling floats and sinks when cooked in the pot, it was first called "Fu Yuanzi". Later, in some areas, "Floating Yuanzi" was renamed Tangyuan.

The Lantern Festival, also known as the Lantern Festival, Xiaozhengyue, Lantern Festival or Lantern Festival, falls on the 15th day of the first lunar month every year and is one of China's traditional festivals. The first lunar month is the first month of the lunar calendar, and the ancients called "night" "xiao". The fifteenth day of the first lunar month is the first full-moon night of the year, so the fifteenth day of the first lunar month is called the "Lantern Festival". According to the Taoist "Three Yuan" theory, the fifteenth day of the first lunar month is also called the "Shangyuan Festival". The Lantern Festival custom has been dominated by the warm and festive lantern viewing custom since ancient times.

The Lantern Festival is a traditional festival in China. The formation of the Lantern Festival customs has a long process. According to general information and folklore, the fifteenth day of the first lunar month has been taken seriously in the Western Han Dynasty. The activity of worshiping "Taiyi" in Ganquan Palace at night was regarded by later generations as the precursor to worshiping the gods on the fifteenth day of the first lunar month ("Historical Records·Yue Shu": "The Han family often worshiped Taiyi Ganquan in the Xin Temple on the first lunar month, and worshiped the Taiyi Temple at dusk at night, It will end tomorrow").