Magic Mirror

There once was a magic mirror which would kill you if you lied to it. It got installed in a public bathroom.
One day, a brunette was touching up her makeup, and said to herself “I’m the smartest woman ever!” She immediately dropped dead.
A few weeks later, a redhead was doing her hair, and said to herself “I’m the prettiest woman alive! ” She immediately dropped dead.
Finally, not long after, a blond was washing her hands, looked up and said to herself “I think,” and dropped dead.