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To play it cool, you have to show that while you enjoy talking to your crush, your life does not revolve around him or her. Here are some ways to make this clear;  Don't talk to your crush every single time you sign online. Talk to him or her every second or third time you sign on. This will show that you're not only going online to continue your conversation. Don't respond to everything he or she says too quickly. If you respond the second he or she asks you a question, this will make it look like you're obsessed with your conversation. If you don't answer right away, your crush will see that you're talking to other people, or doing other things too. Get a secret screen name that no one knows about to see what your crush is doing when he thinks you're not online. This can help you have a better sense of what your crush really thinks about you, and can help you cool it during your interactions.  If you see that your crush isn't online much when he doesn't think you're online, this can suggest that he's online for the same reason you are--to continue flirting! If you see that your crush is just always online, it may mean that he'll talk to whoever--whenever. This may also suggest that he doesn't have a life. Log on to your secret screen name after you say goodbye to your crush. Does your crush sign off right away? This may show that he's only sticking around to talk to you. It's great to show that you think your crush is an engaging, exciting, and fun person, but if you overdo it, you may end up looking too obsessive. Here are some things to avoid:  Minimize your use of "LOL" or "hahaha"--this will make it look like you're trying too hard to be funny. Avoid too many emoticons. This can get annoying and will make you look immature. If your crush doesn't respond right away, don't immediately follow up with a series of question marks or say, "Where did you go???" This will make it look like your world revolves around your crush's answers.
Don't be too eager to talk to your crush. Don't underestimate the secret screen name. Don't be too enthusiastic during your conversation.