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!