Newfags these days! Sage is used to reply to a thread without bumping it. It can be used by entering sage into the email field of your post. Once a thread reaches its reply limit after a lot of posts it can no longer be bumped and will begin to fall to the last page until it no longer exists. Once a thread reaches its reply limit all posts will automatically sage even though you see no proof of it in the email field, this is called auto-sage.