Ruyi Jingu Bang

Ruyi Jingu Bang (Chinese: 如意金箍棒; pinyin: Rúyì Jīngū Bàng), or simply Ruyi Bang or Jingu Bang, is the poetic name of a magical staff wielded by the immortal monkey Sun Wukong in the 16th-century classic Chinese novel Journey to the West. Anthony Yu translates the name simply as "The Compliant Golden-Hooped Rod," while W.J.F. Jenner translates it as the "As-You-Will Gold-Banded Cudgel."

The staff first appears in the third chapter when the Monkey King goes to the underwater kingdom of Ao Guang, the Dragon King of the East Sea, looking for a magic weapon to match his strength and skill. When all of the traditional magic weapons—swords, spears, and halberds weighing thousands of pounds each—fail to meet his standards, the dragon queen suggests to her husband that they give Sun a useless iron pillar taking up space in their treasury. She claims that the ancient shaft had started producing heavenly light days prior and suggests that the monkey is fated to own it. The novel never explains how the pillar was made, only that it was originally used by Yu the Great to measure the depths of the world flood during times immemorial.

The staff is initially described as a pillar of black iron twenty feet in height and the width of a barrel. It is only when Monkey lifts it and suggests that a smaller size would be more manageable that the staff complies with his wishes and shrinks. This is when Sun sees that the weapon is banded with a gold ring on each end, as well as the inscription along the body reading "The Compliant Golden-Hooped Rod. Weight: thirteen thousand five hundred [catties]" (如意金箍棒重一萬三千五百斤). The inscription indicates that the staff follows the commands of its owner, shrinking or growing to their whim, make copies of itself, and that it is immensely heavy, weighing 17,550 lbs (7,960 kg).

When not in use, Monkey shrinks it down to the size of a needle and keeps it tucked behind his ear.

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Wikipedia