When you have a conversation in There, your text appears in a balloon above your avatar's head. As you type, your words become visible to those around you. Hitting the Enter key closes the chat balloon. If you do not press Enter when you are finished speaking, the last thing you say will remain in a word balloon over your head (even when you're walking or running). This feature can be useful for leaving messages. For example, if you need to step away from your keyboard you could type, "I'll be right back", and the message balloon will remain until you Hit Enter to clear it. You can clear the balloons from over your avatar's head by hitting Enter until they've disappeared off the top of the screen.
If you have an IM window up, you can shift between chatting via IM and chatting via chat balloons by hitting the Tab key, or by clicking on chat window/balloon you wish to type in.
You can paste text from another application into your chat balloon by placing it on the clipboard (Ctrl+C or Ctrl+X) and pasting (Ctrl+V) when your cursor is in the chat balloon.