What is the gadget that Nanjing Laomendong shakes and rings?

Nanjing Laomendong's gadget, called "knocker", is a traditional doorbell design. Now, although the knocker is not the main doorbell design, it can still be seen in some traditional buildings such as Laomendong in Nanjing, and it has become a symbol of culture and history. Door knockers are generally composed of two parts: a spherical copper bell and a copper ring hanging on the door. Shake the knocker gently, and the bell will make a crisp sound, thus reminding the host that someone is visiting. The design of the knocker is very clever, and it doesn't need power or buttons. It can make sound only by shaking it gently, so it has been widely used in ancient families.