Mystery Guest Surprises Ellen DeGeneres at The Grove

While best-selling author Ellen DeGeneres made a special appearance at The Grove promoting her new book, "Seriously... I'm Kidding," "Extra" surprised the superstar host by bringing out one of her famous pals... Jack Black!

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Jack, who was at The Grove performing in a concert benefiting The Miracle Project -- an organization dedicated to helping children with autism, read a passage from Ellen's book while he was behind the stage, out of Ellen's sight.

Find out who Ellen thought Jack was when he disguised his voice... check out "Extra's" video!

And listen to Jack sing a few lyrics on the red carpet, gearing up for the "Autism is Awesome" concert with Stephen Stills that benefits the Miracle Project!