A calling card with working planetary gears. Astound your friends, frustrate your nemisii.
(Update: I've moved the gears slightly to the right, reducing the number of cut parts to assemble.)
Cut from card or bristol and assemble like a sandwich. The middle layer needs to be appreciably thicker than the others; I cut it twice and glued the bits together, but I recommend using thicker paper board. To keep the planetary teeth gears from getting caught up in the window edges, bend the gear teeth down (towards the back layer) slightly before assembly. Glue down the middle (sun) gear only. You can put three or four planetary gears inside. It's a bit tricky to assemble; a spare pair of hands helps. Good luck!