There are 3 people standing in front of a magic mirror. The mirror gives you anything you desire if you tell it the truth, but you disappear if you lie.

The first person to talk to the mirror was a very fat brunette. She walked up to the mirror and said "I think I am the thinnest person in the world." and poof, the mirror gobbled her up.

The next person to come up to the mirror was a very ugly red head. She told the mirror "I think I am the prettiest person in the world" and poof, the mirror gobbled her up.

Lastly came the blonde. She walked up to the mirror and said "I think…" and poof, the mirror gobbled her up.