Add Captions to Media You Share via Google Glass
Add a caption in one of two ways.
Tap the touchpad after you tell Glass the contact, Google+ circle, or social network (such as Facebook) with which you want to share your picture or video.
When you share a picture or video from your timeline, you see a message at the bottom of the screen, as shown. This text informs you that you can speak commands, so the other way you can add a caption is to say OK Glass, add caption.