Explaining dual-key crypto with tennis-balls and padlocks

This video illustrates some of the key principles behind "internet security" (actually, dual-key cryptography) using tennis-balls and padlocks. It's does a good job, but it misses out on explaining the hardest -- and most interesting -- concept: private/public key-pairs that have the mathematically provable capability of unscrambling one-another's scrambled messages. I have a long section on this in Little Brother, my forthcoming kids' book about hacking for democracy, and I had to rewrite it more than once to get it right. I'll never forget the moment of satori my editor had when I succeeded in conveying to him the extreme coolth and elegant simplicity of key-pairs over lunch -- then went back to my computer and tried to recapture that in the text.

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(Thanks, Jason!)