Six bamboo skewers
At least eleven elastic bands.
A small plastic pot or bowl.
Something to cut the bamboo - eXacto, scissors, wire cutters*
Ammunition (dried peas, paper pellets etc)
Space to play.
Opponents to battle.
Use three of the skewers to make a triangle. Lash the joints together with elastic bands.
Thread two more skewers into two of the joints of your triangle, then bend them together and lash that joint with another elastic band.
You should now have a diamond-shape, with a bend in the middle (see the third picture).
Cut the last skewer in half, and lash the halves between the two parts of the diamond shape.
Stand the frame on one of the complete triangles - it should look like a mangled pyramid.
The ideal ammunition carrier is a shallow bowl or pouch.
You can use a small plastic pot, such as a trimmed-down film canister, or use a soda bottle cap.
Drill three holes in the edge of the carrier, 120o apart (that is, spread evenly around the edge of the pot).
Fasten an elastic band to each hole with a Lark's Head hitch, then tie the other end of each band to the middles of the skewers that make up the frame. You could even use a round-turn and two half-hitches, but if your bands are short, you can just hook them behind and around the ends of the side-braces.
Take photos of you firing the items out of the catapult.