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Katy Perry's been poking some fun at YouTube star Rebecca Black during her tour Down Under.
Rebecca's song 'Friday' has got everyone talking on YouTube with some people saying it's the worst song ever.
And now Katy Perry's using the song during the part of her show where she performs other 'star's' songs.
The show took place on Friday 29th and Kate said, “I'm so glad that someone wrote a song about Friday — finally!”
You can watch Kate's performance below: