Replying to a tweet allows you to directly respond to something someone else said, and lets you have a conversation in 140-character bursts. To reply to a tweet, click on it to expand, and open the "Reply" box below. You'll also see others' replies to the tweet, if there are any. The username of the person who wrote the tweet will be in the box, as will anyone mentioned in the original tweet. Make sure to take their name out if you'd prefer to leave them out of the discussion. Then, type your message. Note that the @ symbol and username count toward your 140 character limit. As this tweet begins with an @mention, it will appear only in your timeline, the timeline of the person mentioned in the initial @mention, and that of anyone who follows both of you. Additionally, the person who you replied to will be notified the the "Connect" tab. You can expand tweets that are replies in a conversation by clicking "View Conversation" underneath a tweet. Tweets that were sent before and after the tweet you clicked on will appear above and below.