To show how your Chao is feeling, a symbol appears over its head. This is commonly called the emotion ball, or emotiball. The different emotiball symbols mean different things, and combine with the Chao's facial expression to show exactly how it is feeling. These different expressions are explained below.
If your Chao simply has a dot, halo or a spiked ball floating above its head, then it is in its normal state and is probably not thinking of anything in particular.
If your Chao has a large heart floating above it's head, then it is very happy. You may have done something really nice or it might just be doing something it enjoys doing.
If your Chao has a large question mark above its head, then it is thinking about what to do.
Excited / Surprised
If your Chao has a large exclamation mark above its head, then it means it is excited or surprised about something.
Angry / Upset
If your Chao has a large tornado swirl above its head, then it means it is very angry or upset about something. It could also mean that your Chao is tired, which is most commonly caused by waking it up while it is sleeping.
A Chao that is laying down with its eyes closed is asleep. Try not to wake it up, as it will make your Chao upset and tired. It is important to note that Chao sometimes sleep sitting up; be careful not to mistake it for an awake Chao.
In Chao Adventure 2 for the Dreamcast, your Chao refers to the ball as the "thing floating above every Chao's head". He also notes that the "Chao Thing" is "round and kinda squishy" and is "probably the most important thing a Chao has"!