The best time to do this is when you're in the middle of a really great conversation. Drop the request in really casually and make it seem really low pressure.  Say something simple, like "Hey, why don't we trade cell numbers? The moment after a phone number exchange can be a little awkward. Make sure that you don't miss a beat. Keep the conversation flowing so that the number exchange seems completely natural. Before  you send your first message have a plan of what you are going to say or what you want to achieve by the end of the conversation. A simple "What's up (name)?" or "What are you up to?" is a good conversation starter.  If your crush replies that they're watching television, listening to music or playing a game, reply by asking about what they're watching, listening to or playing. Whatever your crush says, you want to be ready with a follow-up question to keep the conversation flowing. Your crush may say something along the lines of, "I'm doing my homework." In reply, you can say something like "We got loads, didn't we? Mine took ages to finish!" Or if your crush doesn't go to your school you could say, "Oh poor you! Ha, did you get a lot?" Tell your crush what you're up to as well. When your crush texts you letting you know what he or she is doing, send a reply like, "That's cool. I'm just checking Facebook" or whatever else you're actually doing at the time. Look for clues in the conversation to see whether the person is enjoying texting back and forth, whether the person has had enough of the conversation or whether you're ready to step it up and ask your crush on a date.  If the answers to your texts are really short or clipped, then you should probably text back something like, "Okay, catch you later," or "See you later alligator." (Cute texting move)Don't read too much into it. The person could be really busy or in a bad mood. Just make sure that you don't make yourself look desperate or needy by trying to push a conversation when it isn't happening. When your crush responds by asking you...

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Write a summary of the previous text in Chinese:
拿到她/他的电话号码。 制定计划！  主动发短信。 注意观察对方的行为。