Now that she's dominated the charts and the radio with "Dark Horse," are you ready for, ready for, Katy Perry's next single?

The singer announced her new song on Twitter on Thursday, and took the popular hashtag #TBT which stands for "Throwback Thursday" to heart. "In honor of #TBT may I present the cover of my NEXXXT single, BIRTHDAY," she tweeted.  She then posted a photo of herself as a young girl, sitting at a table in front of a huge, obviously Photoshopped-in birthday cake, with candles that spell "B I R T H D A Y."

There's another girl pictured sitting next to Katy in the photo, but her face was originally pixilated out.  Who is it?  Katy reveals that in another tweet: "IF you would like to see my sisters amazing face/pose get #BIRTHDAY trending to the top & I will UNLOCK the genius with you... Hahahahahaha."

No word on when "Birthday," another track from PRISM, will be available for download.

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