Bookmark - you can go to 'My Account' -> 'Overview', and there will be a tab listing the bookmarks you've made so that you can easily find the threads again.
Subscribe - you will be sent an e-mail every time there is a new post on the thread. You can un-subscribe any time you choose if you don't want to get the e-mails any more.
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When you are fighting an invisible monster, first throw a bucket of paint over it.