Your blackboard is nicely ordered and a student gives a wrong answer to proceed your lesson. What do you do? Follow the wrong suggestion so the student will see what's wrong, knowing you ruin your blackboard? It's tempting not to do and ask other students until you get a better answer.[br]Fortunately GeoGebra has an 'Undo' button.

Wrong answers can be very interesting for everyone, not only to correct errors but also to understand how the system works, to discover the margins of definitions etc.[br]So an applet with an Undo button in which you can show the result of a wrong answer and easily restore it afterwards, wrong answers can be a gift.