In a previous post on lines, I explained that they create a path for the eye to follow. Lines also convey emotions. As an example: when we draw a happy or sad face the line of the mouth curves up or down. A line curving up signifies joyfulness, while down is gloomy. The trick is learning how to draw an emotion without a face.
Practice boxes are a start in figuring out perceived emotions through lines alone. With Figure 4, I originally tried to make it convey silliness, but more than one classmate said they looked nervous rather than silly or playful. On closer examination, the sharp points and short curves do appear jittery which implies nervousness.