Once again JK Rowling's door is oppened! Yeah!

"You have to wait for Peeves' visit, which takes a few minutes, and some keys will appear in front of the door. If you select the right key you can open the door by dragging it to the lock. You can then see a desk with paperwork and folders. Click on the drawer and pull the magnifying glass over the notepad. You will be asked a riddle - get this right and you will get your prize."

BTW, it takes a while to get the right key, so don't worry if it takes a while. Plus, as a hint for the riddle, it is something you find in ALL Charpter books. Really obvious!