Koa Koa's excellent DIY kits are all about making, learning and playing. This kit lets kids make their very own wooden hand-crank doorbell for their room, complete with gears, bike bell, screws, washers and nuts. They'll learn how gears work, along with the crank, rack, pinion and cogwheels. Once it's built, turn the crank to watch the gears start to move and push the hammer that hits the bell. The doorbell can be painted and there are four holes on the base so that it can be screwed to a door.