# Filters [Filters](https://developer.wordpress.org/plugins/hooks/filters/) give you the ability to change data during the execution of WordPress Core, plugins, and themes. Callback functions for Filters will accept a variable, modify it, and return it. They are meant to work in an isolated manner, and should never have [side effects](https://en.wikipedia.org/wiki/Side_effect_\(computer_science\)) such as affecting global variables and output. Filters expect to have something returned back to them. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://githubichsan.gitbook.io/journal/wordpress-development/hooks/filters.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.