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A while back we told you about the teenage dreamer who shot to fame after performing a Katy Perry hit on YouTube.
Now his idol has made his dreams come true. Katy invited 16 year-old Keenan Cahill to duet with her on stage Philadelphia.
Joined by a cat, the duo performed Katy's hit 'Teenage Dreams', the song which Keenan mimed along to in his bedroom and grabbed the attention of the world.
Take a look at the performance and let us know what you think below...