you39re-pretty
Explanation
The Joke
A young boy tells a girl at school: "You'''re pretty and smart and I wanna marry someone who'''s pretty and smart so we can have babies who are pretty and smart." The girl immediately yells to the teacher: "Mrs. Shelton! Bobby'''s advocating eugenics again!"
The Humor
The comedy comes from the jarring contrast between the innocent childhood context and the serious academic/political term being applied to it. The boy is expressing a simple, naive childhood crush in the way kids do -- he likes a girl and wants to marry her. But when you strip away the cute framing, his reasoning (selecting a mate based on desirable genetic traits to produce superior offspring) is technically a textbook description of eugenics, the selective breeding philosophy that has deeply troubling historical associations.
The word "again" in the girl'''s complaint makes it even funnier, implying this is a recurring problem -- Bobby apparently keeps making these proto-eugenic declarations in class. The girl'''s use of the precise academic term "eugenics" is also humorous, as it'''s incongruous for a young child to use such sophisticated vocabulary.
References
Eugenics is the practice of selectively breeding humans for desirable traits. It was widely promoted in the early 20th century and became associated with horrific abuses, most notably the Nazi regime'''s racial programs. The concept remains deeply controversial in modern ethics and bioethics discussions.