Inception2010

Arthur:
A totem. You need a small object, potentially heavy. Something you can have on you all the time that no one else knows.

Ariadne:
Like a coin?

Arthur:
No, it needs to be more unique than that. [shows her a small red die] This is a loaded die. [Ariadne reaches for it, he pulls his hand away] No, I can't let you touch it, that would defeat the purpose. See, only I know the balance and weight of this particular loaded die. That way, when you look at your totem, you know beyond a doubt that you're not in someone else's dream.