Why a £2 fails (gears)
Following the red arrows, each gear drives the next the opposite sense of motion (clockwise -> anti -> clockwise ...). There are nineteen gears on the back, which unfortunately means that when you get back to the start, you meet a gear that's turning the same way as the first. Cue sparks, broken gears, fire, explosions, death of society etc.
Photo rampantly pinched from www.flickr.com/photos/escaped_monkey/3093421437/ - sorry 'bout that one.