Thanks DonaldDuck. This was originally a Greenfoot demo I did at a SIGCSE conference in Milwaukee; I created the scenario, added the kangaroo, made it jump, and then inverted it. The audience liked it too.
Sorry, I have lost the code. As I mentioned in the comment above I created this scenario as part of a demo at a conference - it took only 10 minutes to write the code and talk about it as I did so. The basic principle of the jumping is the same as in the jumping tutorial video (from the Greenfoot website).
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