Fantastic contraptions, great egg-race
Just thinking of how we construct an essay - wonder whether the game where you build a machine that "does" something - it's a bit like mousetrap...hmm, what on earth is the name, might be a good metaphor to use?
Bit like this
http://www.funny-games.biz/fantastic-contraption.html
and this
http://en.wikipedia.org/wiki/The_Incredible_Machine