To subscribe a thread without contributing

Say, I read a thread and I had nothing to talk about but I still want to read that thread when someone reply. In that case can I subscribe that thread without contributing in it? I think this feature would be nice to have it.

Below 'similar threads' and above 'rate this page', on every threadpage, is a 'subscribe to this thread' link. It's kinda busy down there, but if you hunt for it, it's there.