Sometimes, a topic takes a sudden tangent in an off-topic direction.
Simply go to the topic reply that’s off-topic, and click the “Fork Topic” icon  that appears along the bottom of each reply.
When you click the icon, a popup will appear, allowing you to give the new topic a subject, and assign it to the appropriate forum.
As soon as you click “Fork Post Now,” that particular reply will become a new topic of its own.
In the original topic, the reply will remain, with a link, and a note that it was forked to its own topic.