Raw glutinous rice balls are relatively dense and denser than water. When they are put into water, gravity will dry up and buoyancy will sink to the bottom of the pot. With the increase of water temperature after the dumplings are put into the pot, the stuffing and skin of the dumplings will gradually expand after being saturated with hot water, and the volume will increase, and the volume of boiled water will increase, and the buoyancy will increase. After the dumplings are fully expanded, the buoyancy is greater than gravity, and the dumplings will float. So it seems that the cooked dumplings are bigger than the raw ones.

In addition, because the dumplings are in boiling hot water, the skin of the dumplings is cooked first, and then the stuffing of the dumplings is cooked. At this time, the air wrapped in the dumplings also became hot. When heated, the air would expand, bulging the belly of the dumplings. The belly of the dumplings swelled and became lighter than water, so the dumplings floated to the surface from the bottom of the pot.

Extended data:

Tangyuan, also known as Yuanxiao, Tangtuan and Floating Yuanzi, is one of the representatives of traditional snacks in China, and it is a spherical food made of glutinous rice flour. There are usually fillings, cooked and eaten with soup. At the same time, it is also the most distinctive food of the Lantern Festival with a long history.

According to legend, glutinous rice balls originated in the Song Dynasty. At that time, Mingzhou (now Ningbo City, Zhejiang Province) began to eat a novel food, that is, black sesame seeds and lard were used as stuffing, a little white sugar was added, and glutinous rice flour was rubbed into a circle outside. After cooking, it tasted sweet and delicious, and it was very interesting.

Because this glutinous rice dumpling is floating and heavy when cooked in a pot, it was originally called "Floating Yuanzi", and later it was renamed as dumpling in some areas. However, dumplings symbolize a better family reunion, and eating dumplings means a happy family in the new year, so it is a must-have food for the Lantern Festival on the fifteenth day of the first month. In some areas in the south, people will also get used to eating glutinous rice balls during the Spring Festival, instead of jiaozi.