Twitter’s built-in reply feature, known as @replies (or mentions), lets you target a tweet to a specific twitterer. (Just beware: Unlike direct messages, your reply is public and viewable by anyone who is following you. It is not private.) To send an @reply message, simply click the arrow button to the right of someone’s tweet, which creates a reply tweet with @username at the beginning. It might say, for instance: “@web100 Hey, how about putting me on your Twitter 100 list?” When that user logs on, he’ll be able to see your tweet under the replies tab. This function lets you not only reply to tweets, but also makes it easy to mention other people in your tweets; if you write a message with an @reply (“Check out the Twitter 100 list @web100”), then your followers will have an easy way to link to that twitterer’s account; that’s because clicking on an @reply link sends a Twitter user to that twitterer’s messages. There’s a side benefit, too: Everyone can keep track of who’s tweeting about them by seeing when they’re mentioned as an @reply. As a rule, whenever you want to mention another twitterer, do so with an @reply (by writing @aplusk or @web100).