8 years ago I gave my first presentation on network security. It's a hard topic, and the worst parts were explaining how cryptography works, and explaining software bugs as the buffer overflow or a race condition. Meanwhile, Wired as made an amazing job explaining the formers with an animation. You don't get the cryptography how to (guess prime numbers theory is really hard to draw) but instead you can have a glance at some moments of really bad software.
Update: NSA's site for CryptoKids.