Just so everybody knows this came off of Naruto fever.
You're at the weapons shop buying new shurikens as you used all of them in the last annoying mission that involved scrolls and explosive tags... You asked for 20 shurikens but the dishonest shopkeeper gave you a fake one hidden in the 19 real ones. The fake shuriken looks and feels exactly the same as the real ones, except for a slight weight difference. You've been warned by the jounins about this shopkeeper and they told you a way to find out which shuriken is fake by using a balancing scale. However, you do not know whether the fake shuriken is heavier or lighter than the real ones.
Try figuring this one out! When enough people guessed then I will tell the answer.