Ruyi Jingu Bang

It is an iron rod whose size changes ruyi 如意 "as-one-wishes," which Sun Wukong obtains from the undersea palace of the Dragon King of the East Sea. It is immensely heavy (weighing "13,500")(8,100 kg/17,820 lbs); in modern measurements, it is slightly heavier than 8.5 metric tons.

It has the power to change its size, multiply itself, and fight according to the whim of its master.

When it is not in use, Sun Wukong shrinks it to the size of a sewing needle and keeps it behind his ear. The novel describes it as being made of "black iron" (烏鐵 wūtiě) with two gold bands (Chinese: 金箍 jīngū) round it near the ends. In adaptations, it is often depicted as being either red, black, or silver in color, or as a solid gold or copper staff.

Sun Wukong commanded it to shrink and grow. In others, the dragon king simply believed that it was too heavy to use as a weapon and didn't expect him to take it.