Unregistered users can watch most videos on the site, while registered users are permitted to upload an unlimited number of videos. Some videos are available only to users of age 18 or older (e.g. videos containing potentially offensive content). The uploading of videos containing defamation, harassment, commercial advertisements and material encouraging criminal conduct is strictly prohibited. Related videos, determined by title and tags, appear onscreen to the right of a given video. In YouTube's second year, functions were added to enhance user ability to post video responses and subscribe to content feeds.