Wicked Science: Koala in the Mist. Garth lopes off with Elizabeth's shrink ray solely to make a dessert larger because he can't afford anything else, only for it to end up in Jack's hands - and soon there's a giant Koala bear in the woods next to the school... This was another really good episode, with lots of surprises and laughs. Jack is now essentially becoming the primary antagonist of season 2, basically threatening both Toby and Elizabeth at the end of the episode. Toby initially blames Elizabeth for the chaos, only to feel (rightfully!) guilty when he realises it was Jack all along. Elizabeth lets him off with a flirty "Maybe you can make it up to me sometime".
