How about this?
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1232948
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They're not round cakes, but I think a giant blue cell phone would be a cool cake for a teenage girl. It seems like a lot of young teens love their phones and are into texting. You could write the birthday message as a text on the screen.
Text:
OMG!
MIA IS 13!
H-BDAY!
XOXOXO
I just made a big baseball theme cake for a 13 year old girl.....its what she asked for....I was hoping for girly, but well thats what she wanted.
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