In the episode, Cartman puts a picture of Kenny's butt on a milk carton as a prank, but loses his ability to laugh when a couple arrives in South Park with buttocks instead of faces.
It is picture day at South Park Elementary, and as a prank, Kenny wears his parka upside down and stands on his head with his legs through the sleeves so that his buttocks show through his hood.
Cartman's description of him includes features with obvious double meanings, such as a "winking brown eye", "blonde hair", and "rosy cheeks".
The photo draws a response from a couple in Wisconsin, Martha and Stephen Thompson, who lost their son some years earlier, and shockingly have buttocks in place of their faces.
Stan and Kyle conclude that he could not laugh before because he felt guilty for making the Thompsons believe that their son had been found – which Cartman vehemently denies.
[1] "How to Eat with Your Butt," along with the thirteen other episodes from South Park: the Complete Fifth Season, was released on a three-disc DVD set in the United States on February 22, 2005.