In mailing lists, message boards, and group emails, lively discussions often arise in individual messages. As these discussions grow longer, the subject may change. Usually, it has nothing to do with the subject of the original message. You can change the subject line of a message thread when it becomes clear that the subject of the discussion has changed.

Depending on where you are, you may be able to change the subject by typing a new subject in the subject line when you write your reply. But this may not be the best course of action.

Instead of changing the subject, you can make it clear that you are continuing an old topic and not starting a new one. You can do this by using the subject line of the old topic in the new line.

If the original subject was "New Cloud Shape Discovered" and you want to change it to "The Most Beautiful English Umbrella", the new subject line might be "The Most Beautiful English Umbrella (was: New Cloud Shape Discovered)". You can, and probably should, shorten the original subject line.