Some info and a short description of this South Park episode on South Park Zone:
This south park episode is the 75th of the Comedy Central series South Park. It originally aired on November 14, 2001.
As it is picture day at South Park Elementary, Kenny wears his anorak upside down
It is picture day at South Park Elementary, and Kenny wears his parka upside down and stands on his head with his feet through the sleeves so that his behind show through his hood.
When Cartams prank of putting Kenny's behind on a milk carton prompts a family with behinds for faces to come to South Park in search of their long lost son, Cartman "blows a funny fuse" as he sees the funniest thing ever. Meanwhile, poor Butters gets grounded by his weird parents for making a supposedly funny face in his school photo.
South Park Spoiler Alert!
(The complete plot for this South Park Episode)
It is picture day at South Park Elementary, and Kenny wears his parka upside down and stands on his head with his feet through the sleeves so that his buttocks show through his hood.When Ms. Choksondik says that one boy spoiled his photo and will not get it back, Kenny thinks she is referring to him. It turns out that she is referring to Butters, rather than Kenny, even though Butters looked normal in the photo. His parents, irrationally convinced that Butters' normal face is some sort of silly face, promptly ground him and force him to wear a brown paper bag for making "silly faces".
To compound this trick, Cartman then turns in Kenny's photo to a milk company, which places Kenny's photo on a milk carton. Cartman's description of him includes that he has a "winking brown eye", "blonde hair", and "rosy cheeks".
The photo draws a response from a couple, Martha and Stephen Thompson, who lost their son some years earlier. They suffer from a fictitious congenital condition known as "torsonic polarity syndrome" or "TPS" which has them born with their faces appearing as the human buttocks. With the syndrome, however, they still retain all normal functionality of their faces.
The Thompsons come from Wisconsin, and the name of their son, Tommy, is the same as that of former Wisconsin governor Tommy Thompson, who had just left office to become Health and Human Services Secretary under George W. Bush.
When the Thompsons appear looking at Cartman's house for Kenny, he is stunned and can no longer laugh, believing he has seen something so incredibly funny, that nothing else will ever be able to make him laugh again. Cartman referred to this as "blowing his funny fuse".
Meanwhile, Cartman does whatever he can to try and make himself laugh. He goes through a list of things like seeing a raunchy movie, and talking to Jimmy for his stand-up routines. When all fails, Cartman is shown in his room holding a pistol. He writes a letter to his mother, explaining that he cannot face his friends without a sense of humor, places the gun in his mouth, and takes a bite. It is at this point he asks for more chocolate guns.
At the end of the episode, the South Park milk company officially announces that Ben Affleck was the missing son of the Thompsons. When Cartman finds out, he starts to laugh again. Stan and Kyle decide that he could not laugh before because he actually felt guilty for making the Thompsons believe their son had been found – which Cartman vehemently denies.