Blonde in a Bar

One day, a blonde, a redhead, and a brunette walk into a bar. At the back of this bar, there exists a magic mirror that sucks up anyone who tells a lie in front of it.
All three girls walk up to the mirror to say something.
The redhead goes first, and she says, "I know I'm the most beautiful girl in the world", and she is sucked in.
Then the brunette steps up to give it a try. She says, "I know I'm the most beautiful girl in the world", and she gets sucked in.
Finally, the blonde walks to the front of the mirror. She then says, "I know-", and she gets sucked in.