Depends on how you have the settings rigged up. Simply browsing a thread won't add it to your watch list. But there is an option that you can set for the software to automatically add threads to your watch list based on if you reply. Just click on 'Preferences' under the drop down menu in your username in the upper right. Should be a tick box for 'automatically watch threads I start or reply to'. Other wise, you need to manually add the thread to the watch list.