How to Make a Sling for a Lego Trebuchet

Choose a material for your trebuchet sling that's elastic in nature but not overly elastic; you'll need some rigidity so that the sling can hold the weight load but not droop to the ground. Elastic hair ties or industrial-grade rubber bands make for good choices, but take the size of your Lego machine into consideration: Longer bands will result in errant trajectories, and shorter bands will shorten the shooting distance.

Snip one edge of the rubber band or hair tie so that the band/tie can be laid out in a flat, straight line. Measure the band: You're looking for the middle inch of the band, the point where you'll want to attach the actual sling. It doesn't have to be exact, but too far to either side may result in missed throws.

Cut a 2-inch-by-1-inch piece of leather to make a small rectangle, which will be the basis for the Lego sling.

Lay the band or hair tie lengthwise across the rectangle. Position the leather so the band lies in the middle of the rectangle's length, and fold the upper-left corner across the band over of the lower-left corner. Generously dab the inside of the corners with super-adhesive gel, and press down on the corners until they're secure. Repeat for the right corners of the leather rectangular sling.

Attach the open ends of the band --- the point where you snipped the band --- over the trebuchet's arm. Secure the ends to each other and to the machine's arm by tightly wrapping electrical tape around the ends and the arm. The leather sling should be hanging down from the arm.

Dab adhesive on one side of the small refrigerator magnet, and glue the magnet to the underside of the leather sling. Pinch the magnet and the leather together to ensure a tight seal is met. Allow an additional 30 minutes or so to make sure the adhesive dries before loading the sling.