How to Cook for Forty Humans
Wikisimpsons - The Simpsons Wiki
How to Cook for Forty Humans is a cookbook that the Rigellians keep in the kitchen of their spaceship. The book is a guide for preparing meals for large groups of humans. Serak the Preparer referred to it while preparing meals for the Simpson family when they were on board the ship.
The Rigellians abducted the Simpsons from their backyard and said they were taking them to Rigel 4 for a feast. The aliens treated the Simpsons very well, providing them with all the entertainment that Rigellian technology had to offer and with sumptuous meals prepared and served by their cook, Serak the Preparer. While the rest of the family enjoyed the food and entertainment, Lisa began doubting the aliens' motives, particularly when Serak told Homer that Marge was "quite a dish" and when Kang said that the Simpsons would be the guests of honor at the feast (which elicited chuckling from all of the aliens).
Now suspicious, Lisa snuck away and snooped around while the aliens weighed Bart and Homer (and congratulated Homer for his having gained weight). Lisa soon ended up outside the kitchen. She peeked in and saw Serak working at the stove, adding spices to a pot, saying "This will give the humans the perfect flavor" and licking his lips. When Serak left the kitchen, Lisa went in, found the cookbook and read the title on the cover: How to Cook Humans.
Lisa then confronted the aliens, accusing them of planning to eat the Simpsons and showing the book as proof of her claim, which shocked both the family and the aliens. Kang blew a little dust off the cover, showing the title to be How to Cook for Humans, to the family's relief. Still suspicious, Lisa blew more dust off the cover and the title then appeared to be How to Cook Forty Humans, shocking the family again. Kang, however, blew off the last of the dust, revealing the book's real (and finally undisputed) title to be How to Cook for Forty Humans.
Hurt from the accusation, disappointed with the Simpsons' distrust, and disgusted with their eating like pigs, the aliens took the family back to Earth and told them that they had forfeited an idyllic existence on Rigel 4 where they would have been treated as gods. After the aliens left, Lisa, waxing philosophical, said, "Truly there were monsters on that ship, and truly we were them."
 Behind the Laughter
 Title Reveal Sequence