Sounds like a scene from "A Bug's Life"
"Pretend this is a seed."
"But it's a rock."
"No, imagine this is a seed. One day, it'll..."
"But it's a ROCK!"
For those of you who haven't seen "A Bug's Life", go rent it right away. Funny stuff and terrific animation. Be sure to watch the "out takes" on the bonus material if you get the DVD version.
