Let's just say, as you can see for yourself in this exclusive sneak peek, it isn't any of Silverstone's sartorial choices that has Joan feeding Kelly Osbourne a piece of hamburger...with her own mouth!
"We're a very happy group here, and we're like family, too," the comedy icon said, already beginning to crack up.
How far is Joan willing to take the joke? Watch the clip now and find out!
Fashion Police airs Friday at 10 p.m. on E!