Probably the best episode ever! I happened to stumble upon this episode on late night TV last night and it is priceless. I wish I could find some illegal copy online but the kind folks at FOX have taken them all away. So I offer you this bio while I save up to buy Season 7 on DVD

Lisa realizes that it's wrong to eat animals after a long petting session with a baby lamb at a local zoo. Swearing off meat turns out to be a controversial decision for Lisa, since everyone around her seems to support the eating of flesh. At school, Lisa must sit through a politically slanted pro-meat film and eat only hotdog buns for lunch; at home, she has to put up with Homer's plans for a meaty barbecue party. After fighting about her beliefs with her family, Lisa runs away to the Kwik-E-Mart, where she discovers a kindred spirit in Apu, who is not only a strict vegetarian, but a vegan as well. Lisa also meets two other vegetarians, Paul and Linda McCartney, who explain that while it's important to stick to her convictions about vegetarianism, she must also tolerate the beliefs of others. Knowing that Paul and Linda are right, Lisa makes up with Homer and they accept each other's different points of view. Guest Stars: Paul and Linda McCartney as themselves

I have found some short clips online, watch them here
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