Vial of Osmium

Vial of Osmium is an item on Monster Carnival.

Role on Monster Carnival
After you try the weight-guessing game at the carnival, you go to Dr. Dan to ask for something "small but heavy" to trick the weight-guesser. You find the vial of osmium amongst his mail. After this, you go back to the weight-guessing game and cheat the weight-guesser. The vial of dense osmium adds a lot of weight to you, causing the weight-guesser to misguess your weight.

Trivia

 * Osmium has the highest density of any element.