The kids on your block have gathered. The hoses are getting unravelled. You have three minutes before the first water balloons start flying. Four minutes tops if Tommy still can't tie knots without his mom's help. Susie and Jermaine come running to your side. "We need a slingshot" they say. You have just latex tubing and duct tape.
No problem. :)
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Step 1: Make a Tube Loop
Start with about six feet of latex tubing, which you can get from most hardware stores. Tie the two ends together with a double knot, and then cut off the extras.
2:40 to go. The older 4th graders across the street are hogging the hoses.
Step 2: Duct Tape Side Handles
Roll your duct tape around the knotted part to make a handle and keep the knot together. Then spread out your loop and find the opposite side. Use the duct tape to wrap and make a matching handle.
2:10 to go. Tommy is calling for his parents to help him with knot tying.
Step 3: Make a Duct Tape Pouch
Lay out five strips or so of duct tape face up and next to each other overlapping slightly. Put the mat underneath your tubing in the middle and fold it over so it sticks to itself. Do the same on both sides. Scrunch the top and bottom of the pouch a little bit along the tubing to make it more pocket-shaped.
1:10 to go! Turns out Tommy's mom, dad, and grandma are all balloon knot-tying wizards. Susie and Jermaine kick into gear.
Step 4: The Pullback Handle
Lay down two long strips of duct tape face up, and then put two shorter strips over them. Use the remaining sticky sides to wrap the handle around your existing pouch. Now you have a handle to pull your balloons back with. To make it sturdy, make it long enough that the two ends overlap.
10 seconds to go! Susie and Jermaine are in position. The 4th graders are sneering. Tommy's grandma has a Super Soaker and a firehose. You are calm. It's time.
Step 5: Sling Some Balloons! (and Baked Goods)
It's launching time! Tell your two friends to stand side by side holding the side handles with their hands up in the air. Put the water balloon in the pouch, and pull back by the handle. If you want maximum distance, use your remaining ten seconds to read this primer on the physics of projectile motion.
Also, when you'd like to start wooing people instead of watering them, you can put muffins or donut holes in as well. They fire great! Make sure to eat them after. Firing food into the air is great. Wasting it isn't.
If for whatever reason your friends get called in for dinner before you do, the one-person method is demonstrated in the last photo. Sure you'll look awkward, but you'll be efficient.
Have fun, be safe, make sure people consent to being launched at, and keep exploring. Can't wait to see what you make!