(topic: Codes) - In the watery depths of cybersite Aquari-yum, the kids and Hacker go in search of a giant cyber-slug, who feeds on pure energy. The kids and Digit travel underwater in two separate vessels, but Hacker has a listening device that taps into their communications system, allowing him to overhear their plans - and track every move they make. The kids outsmart Hacker by devising a series of codes so they can send messages back and forth in secret - even though Hacker can hear what they're saying. But unless the kids can reach the slug first, Hacker will use the slug to attack Motherboard's circuitry and disable her once and for all! The Big Idea: You can use a code (a communication system that substitutes one thing for another) to send top-secret messages as long as your code has an adjustable rule that can be reversed to read the message.