sodomy
Explanation
The Joke
A child asks his father, "Dad, what's 'sodomy'?" The father, looking stern, declares it is "a very bad thing" and defines it as "whenever you put your genitals somewhere that is unnatural for them to be." The child, taking the definition at face value rather than understanding the intended sexual meaning, innocently asks "Like in an airplane?" The father, either not catching the absurdity or genuinely agreeing with the overly broad definition, confirms: "Exactly."
The final panel shows the father outside, shaking his fist at the sky and yelling "Perverts! Perverts!" at airplanes flying overhead, having fully committed to the idea that anyone whose genitals are in an airplane is committing an unnatural act.
The Humor
The joke operates on multiple levels. First, there is the classic comedy setup of a child asking an embarrassing question and a parent fumbling the explanation. The father's definition of sodomy is deliberately vague and overly broad -- "putting your genitals somewhere unnatural for them to be" -- which, if taken literally, could describe almost any situation where a human body is in a location that did not exist in nature. The child's logical application of this definition to airplane travel is technically correct given the father's wording, which exposes how poorly thought-out the explanation was.
The final panel escalates the joke from a misunderstanding into full-blown lunacy, as the father adopts his own bad definition as a sincere worldview and begins raging at airline passengers. This satirizes how moral outrage, particularly around sexuality, often relies on vague and poorly defined principles that, if applied consistently, would lead to absurd conclusions.