There's a magical mirror.
Say what you think about yourself in front of it. If you're wrong, you disappear and go POOF! Three people test it out.
The first is a brunette girl.
She says to the mirror, "I think I'm the coolest person in the whole world."
POOF!
The second person is a red-head.
She says to the mirror, "I think I'm the hottest girl in the world."
POOF!
The third was a blond.
She says to the mirror, "I think--"
POOF!