La pluma de IQ

To make this puzzle, drill a small hole through a new pen just at the top. Thread a piece of string through the hole and tie a knot in it to create a loop that is shorter than the pen.

Next, find an old shirt with buttonholes large enough for the pen to fit through freely. Place the loop around the buttonhole as shown in the illustration. Pull the fabric surrounding the buttonhole up through the loop until the tip of the pen can go through the buttonhole from underneath. Then pull the pen through and the loop will form an overhand knot around the buttonhole.

Now that the pen is attached, all you have to do is remove it and you've solved the puzzle. Good luck!