Why use green screen?

Weather reports
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On location without the hassle
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Storytelling
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More information about using green screen for digital storytelling at Jason Ohler's website

What are some possibilities?

"Superhero" flying
Climbing
Student entering images (ie artwork)
Students interacting with things of disproportionate sizes (students become giants, become tiny)

Resources:

Chroma Key on Wikipedia: http://en.wikipedia.org/wiki/Chroma_key
How to use Green Screen in iMovie 09: http://www.iclarified.com/entry/index.php?enid=2984
Jason Olher Reading and Writing 2.0 http://www.jasonohler.com/pdfs/CIC1107ReadingWriting.pdf
Download a storyboardsheet.pdf template.
Look at how a Kindergarten teacher used Green Screen and Literacy Scholastic.com blog