Short animal calls like this moo have become a staple soundboard clip because they need no context to be understood. The name combines the number 13 with the word moo, and the clip itself is just a single call with no speech or music behind it. Online, creators reuse brief farm sounds in place of a spoken reaction, letting the animal noise comment on a scene. Its popularity comes from that simplicity, it can be added to almost any light-hearted edit and viewers will get the joke immediately without any explanation needed at all.