AFI #004 Judy Garland as Dorothy Gale from The Wizard of Oz (1939)

Dorothy: [has just arrived in Oz, looking around and awed at the beauty and splendor] Toto, I've a feeling we're not in Kansas anymore.
Dorothy: [after a pause] We must be over the rainbow!
[a bubble appears in the sky and gets closer and closer. It finally lands, then turns into Glinda the Good Witch wearing a spectacular white dress and crown, holding a wand]
Dorothy: [to Toto] Now I... I know we're not in Kansas!

- Judy Garland as Dorothy Gale from The Wizard of Oz (1939)

The bolded line is ranked #4 in the American Film Institute's list of the top 100 movie quotations in American cinema.

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American Film Institute's list of the top 100 movie quotations