Making Faces
Making Faces (which initially aired in the United States of America on September 22, 1987), is the eighth Simpsons short.
Bart, Lisa and Maggie are all sitting in a room. Each young family member grimaces in turn, before Marge walks into the room with the warning that their faces would stay like that if they continued.
Lisa, initially concerned her mother was telling the truth, is helped by Bart, who says that Marge would not have said something that was not true. Despite this, they still are happy.
The next scene consists of them each trying to make scary faces, but then agreeing that the scariest face of all was that of Marge when she catches them doing the same thing she initially warned them against.
They soon realize that, as Marge had warned them, their faces have actually stuck in place. The episode ends.