I checked the FAQs to look for an answer without any luck. Forgive me if it is there and I overlooked it. I recently asked a question with the title: Causality in a gedanken experiment on the hydrogen atom, and John Rennie commented on it. In my response I simply put his name in the text. I mean it was like "Yes, John Rennie ..." instead of "@John Rennie Yes ..." I have observed that the latter is pretty standard on this forum and I would like to learn how to do that. Could someone please tell me how that is done?