Gold Pen

From the Super Mario Wiki, the Mario encyclopedia

The Gold Pen is an item in Monster Mix-Up. It is made entirely out of gold and resembles a simple fountain pen. Mario obtains it from a Coin room after turning the nearby Chomp-Munchers into coins with a P Switch. As soon as he spots the Pen glistening on top of a block, Mario immediately jumps for it and pockets it. It is later used as the amalgamation of the Mushroom King and Elmo leads Mario and Toad around the castle: the group ends up at a door with a trick lock, and the amalgamate explains that the door opens once the entire picture is traced without removing the pen from the lock. He also says that Gold Pens are the only writing utensil that work. If Mario does not have the Pen, Mario and Toad fall into an underground cavern. If Mario starts tracing from the incorrect place, Mario and Toad fall into an underground sewer system (without losing the Pen). If Mario solves the puzzle correctly, the doors immediately swing open, and the group continues onward.