Why did Mazu's daughter-in-law keep her mouth shut?

It is a custom for Mazu to put a needle in a child's mouth. In Chaoshan, Fujian, Taiwan Province and other areas of Guangdong, Mazu is the common belief of local fishermen, boatmen and transporters. In folklore, Mazu will save people from danger, and Mazu's children are the medium between gods and people. In order to show respect for the gods, Mazu girl will pierce her mouth with a steel needle when offering sacrifices or "inviting fire" (to the holy grail), indicating that she has no evil thoughts and worships the gods wholeheartedly.