How to Make a Far Throwing LEGO Catapult

Create a base using three layers of one-prong-wide LEGO strips on a flat 8-inch by 8-inch LEGO base plate. Set the base walls parallel to each other with four rows of prongs between them, each row an equal distance in from the edges of the plate.

Construct two A-frames using two 2-inch-long single-prong LEGO struts with holes connected with an arm pin at the top and another 2-inch strut at the base to make a triangle. Connect another 2-inch strut at each base corner of the triangle and secure with pins to make the "A" shape. Mount the A-frames to the three-layer walls created in the previous step with pins.

Attach two single-prong LEGO struts with holes to each end of a two-prong by 16-prong plate to create the catapult arm. Secure with an additional two-prong by 16-prong plate underneath to sandwich the struts.

Attach a lead fishing weight to one end of the LEGO catapult arm by threading copper wire through the outermost strut holes. On the opposite end of the arm, thread a 3-inch piece of elastic string through the strut holes and tie a key ring to the loose end of the string.

Cut a small strip of T-shirt, approximately 3 inches long and 1 inch wide. Stitch one end together so the T-shirt becomes a cup-like pouch to hold the projectile. Wrap the sewn end around the key ring and stitch in place.

Pull the T-shirt scrap away from the catapult, stretching the elastic line and raising the lead weight. Load the T-shirt with a marshmallow, pull back tightly and release to launch the marshmallow up to 30 feet.