Student mistakes correspond to dropping an object in the wrong box (i.e., using the wrong attribute in a computation). In this case, the system shows her a plume of gray smoke (Figure 3f). On the other hand, if the student drops the right object in a box (i.e., she uses the right attribute), the effect will depend upon whether the evaluation process has been completed or not. In the first case, the box will emit the aforementioned multicolor beam; in the second case, the box will remain inactive. Additionally, the student can be forced to restage a level if she exceeds a certain number of errors (the details of this behavior can be also customized by the instructor).