How to convert short messages into TXT files

I don't know what brand of your mobile phone is, and I don't know if there is a short message copy function on it. If there is, you can open the SMS editing page, and then open the options or menu, which has the function of copying. You copy the short message, and then create a new text document in the memory card, just paste the copied content, so there will be the short message content in TXT format. It is the same process to convert TXT into SMS and send it to mobile phone.