About this "The Simpsons" episode:
The Simpsons - Lisa the Simpson is the 17th episode of season 9 and number 195 overall in the series.

Soon after being unable to solve a brain-teaser that several others at school were able to solve, and forgetting some of her day to day tasks, Lisa learns from Grampa that the Simpsons have a long history of losing their intelligence in late childhood. Lisa soon realizes that she cannot escape genetics, and accepts that she will soon be very unintelligent. After realizing the importance of sharing what you can with the world while you still have the chance, she lies her way onto a news broadcast to deliver a message on treasuring your brain. Homer sees this and, upon learning what Abe told her, attempts to disprove the theory of the "Simpson gene" by gathering every Simpson in the area together to showcase their merits. Unfortunately it is quickly apparent that Homer did not research this project much, as many of his guests prove to be simpletons, deadbeats, and stooges. Lisa's salvation only comes when she begins to meet the Simpson women, including a doctor who submits that the defective "Simpson gene" is located only on the Y-chromosome. Re-validated, Lisa celebrates that she is once again herself, an event aptly crowned when she finally manages to solve the brain-teaser that so plagued her.