There's a 15-year-old named Keenan Cahill who suffers from a disease called Maroteaux-Lamy Syndrome (--pronounced mar-oh-toe la-MEE).  It caused him to stop growing when he was around eight years old.  You might have heard his name before, because he posts videos on YouTube where he lip-syncs to popular songs.  He does a new one every week, and the one he did for KATY PERRY'S "Teenage Dream" in August has gotten over 15 million views. 

But his newest video is for the 50 CENT song "Down On Me" . . . and 50 Cent actually PARTICIPATES in it.  They recorded it the other day when Keenan was a guest on "Chelsea Lately".