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If the girl gave you her number and you want to start texting with her, a good starting point is to text about something you will both laugh about. Maybe there's an inside joke or something that you both have in common that you can message about. Stay away from texting something boring like, “How are you?” This will probably only get you a boring answer and it won't engage her very much. Send a text with specific details about the last time you were together in person. For example, you might say, “I loved you in that red dress the other day.” She'll feel special that you remembered these details. Sending 20 texts in different parts of the day will likely be overwhelming. Stick to 3 or 4 text sessions where you send her a quick message and respond a few times to her texts. Texting is a good supplement to other interactions, like meeting up in person or talking on the phone. Texting can help you flirt, but it shouldn't be the only way you flirt with someone. Even if you're shy, you should get to know this girl by talking on the phone or going on a casual outing together. A girl might be at work or busy doing something and can't always immediately text back. She might not be into texting and would prefer a phone call. Get a feel for how she responds to your texts and don't take it personally.
Text an inside joke. Mention details. Don't text too much. Don't rely only on texting. Don't get upset if she doesn't text back.