In a few days we are dealing with behaviour in class. I came across this video, which could cast some light on it. It focuses on the importance of body language and how it could affect the way others see us. I do recommend you to watch it. You might not have realised what you are saying to others by the way you sit or move your hands.
There is no exercise to do, just listen and enjoy the talk. If you find it difficult, activate the subtitles.