You choose whether to follow a topic in your personal settings.
By default the 'follow this topic' box will be checked when you make new content or comment/reply to a topic.
To change this Go to Manage > Your Account > Personal Settings. Under Notifications, choose which options you wish to enable. Click "update settings."
Then you can choose to follow any topics by manually checking the 'Follow this topic' box when you create your topic or comment/reply.
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