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Mrs. Krabappel had given Bart a month's detention and taken away his yo-yo because of a yo-yo stunt that had killed the classroom fish. Bart was angry about it and looking for opportunities for revenge. When Mrs. Krabappel stepped out of the classroom during one of his detentions, he rummaged through her desk and found his yo-yo (which he took back) and a personals ad that she had placed.
Bart decided to get revenge by sending a prank reply to the ad. He started by saying that he liked holding hands and dinner by candlelight. Also, because he knew that Mrs. Krabappel was annoyed by the yo-yo craze that had swept through Springfield Elementary School, he added that he hated yo-yos in order to further appeal to her. Finally, he needed a name to sign to his reply, and decided on "Woodrow" after he looked around and spotted a portrait of former U.S. President Woodrow Wilson. The letter was a total success as Mrs. Krabappel immediately fell in love with Woodrow when she got the letter.
After that, Bart carried out a chain of romantic correspondence with Mrs. Krabappel, using Woodrow as his alias. For material, he read his parents' old love letters, watched romantic movies on TV, and listened to Mrs. Krabappel to get an idea of what she wanted in a man. For example, after Mrs. Krabappel sent Woodrow a sexy photo of herself and Bart overheard her saying that she wanted a man who could fix her car, Bart wrote in his next reply:
Mrs. Krabappel read the letter in her bubble bath. At the last line, she ecstatically submerged herself beneath the bubbles.
When it came time for Bart to send Mrs. Krabappel a photo of Woodrow, Bart flipped through a book of NHL stars, spotted a photo of Gordie Howe, decided she would like it and sent it to her. Mrs. Krabappel was pleased with the photo, which she put in a frame and kept on her desk at school.
When Mrs. Krabappel suggested they meet for dinner and then go to her apartment for some "home cookin'", Bart decided the prank had gone on long enough. He made a date to meet Mrs. Krabappel at The Gilded Truffle, planning to end the prank by standing her up. While Mrs. Krabappel went to The Gilded Truffle to meet Woodrow, Bart went to the Aztec Theater to see Ernest Needs a Kidney. Bart enjoyed the movie, but afterwards he felt remorseful when he walked by The Gilded Truffle, saw Mrs. Krabappel sitting alone at a table, and realized how embarrassed and hurt she was that Woodrow didn't show for the date. Realizing he was in over his head, he turned to his family for help.
With help from Marge and Lisa (and even a little from Homer), Bart was able to write Mrs. Krabappel a goodbye letter, where he said that Woodrow had to go away and couldn't say why, but still loved Edna. The letter succeeded as Mrs. Krabappel was sad that Woodrow was leaving, but happy with how things ended.
 Behind the Laughter