You sign up to follow someone's tweets. That person, IF he is aware of you, can follow yours. However, you cannot reply to a specific tweet.
"Tweets are text-based posts of up to 140 characters displayed on the author's profile page and delivered to the author's subscribers who are known as followers. Senders can restrict delivery to those in their circle of friends or, by default, allow open access. Users can send and receive tweets via the Twitter website, Short Message Service (SMS) or external applications."
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