Not really a gadget, but here’s a shot of a craft stick catapult that I built with my Webelos den last week (I’m the Den Leader). We used the sticks, rubbber bands, and plastic spoons to build catapults, and then had a distance/accuracy contest to see who could best launch little mini marshmallows across the room and into a wastebasket target.
The initial design had a relatively small rubberband on the bottom, which controlled the force of the launch. It was pretty weak, so I beefed mine up with a bigger band. I was worried that the structure wouln’t take the extra tension, but it held up.
The extra resistance from the bottom band meant that the upper band (which you can’t really see in this picture), which held the spoon and acts like the axle, needed to be beefed up, too.
With those two modifications, the catapult became a formidable marshmallow launching machine, though accuracy was low. Coupled with the PVC marshmallow shooters we made last year, I’m amassing quite the soft white sugary arsenal. 
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