Katy Perry has made a video performing with Keenan Cahill, whose cover of 'Teenage Dream' has received more than 40m views since August 2010.
The video by the 16-year old from Illinois 'went viral' last September after Katy tweeted "I heart you".
Last Friday (night) Keenan attended her tour in Philadelphia on Katy's invitation, and the pair filmed a video singing together to 'Teenage Dream'.
A large purple cat, one of the characters from Katy's current 'California Dreams' tour, provides back-up.