....don't bother.
I've just wasted a fiver on it (took dd thirty seconds once my back was turned to frisbee her spag bol onto the curtains) and when i bitterly mentioned this on my blog i had five or six people comment that their children had easily defeated them as well. (one woman enjoys booby-trapping her husband's plate with it, but her child can tackle it no problem).
so there you go, i just saved you a fiver. grrrrrrrrrrrr