Inspired by all the creative minds here on instructables, I made a ping pong catapult for my kids to play with.

Materials used:
Several 1/2 inch x 1/2 inch dowels (about 7 or 8). The longest piece is 12 inches long.
2 paint stirrers
several zip ties
2 aluminum pipes (1/4 inch in diameter) 4 inches long. These were from an old wind chime i had laying around. Metal isn't necessary. Sturdy plastic might work (like the straw found inside a windex bottle)
8 small screws that i salvaged from junk in the garage.
1 tiny cup that sugar free drink mixes come in (like the 2 quart size crystal light powder)
various rubber bands

Tools used:
staple gun
screwdriver
sturdy scissors or wire cutters
drill or dremel

optional tools used:
glue gun

Ammo:
PING PONG BALLS! (or cotton balls, or wadded up paper, or...?)

Step 1: Build the Base

step one:
build the base.

The length of the base turned out to be the length of the longest dowels (i'm too lazy... er... i mean, i was trying to be efficient!)
The width of the base turned out to be the width of the small aluminum pipes (from the wind chime)
So, the cross member dowel measures 4 inches, making the outside width of the base 5 inches (4" + 1/2" + 1/2" = 5")

I put the cross members at the ends of the long members to make it look neat and trim, but this proved to be problematic, since the screw split the dowels, forcing me to drill holes instead of simply screwing in the screws. if you move the cross members in a little, you might be able to skip the drilling.

Step 2: Build the Catapult Axle

next, build up the cross member to provide support for the axle. The length isn't a big deal, just use some scrap left over from cutting the cross members. I simply stapled these on. it takes a little finesse to get the staple gun in the right position. careful! watch your fingers!

take a dowel that is at least as long as the long members of the base - this will be the catapult arm. drill a hole in its end - through which you'll put the pipe that will become the axle. i zip tied the dowel above and below the drilled spot to keep it from splintering. (looking back on this step, i suspect you could just as easily zip tie the dowel to the axle without any need to drill a hole.)

center the dowel, then zip tie both sides to keep it from moving cross-wise on the axle.

hold the pipe in place while you screw the screws into it. the screws are snug to the wood, but not the pipe.

The screws i used were well over 1 inch long - providing a pivot point for the wind chime pipe to rotate on. That is to say that the pipe (the axle) is not secured to the catapult. it spins freely on the screws that protrude inside the pipe.

Step 3: Build the Tower

the dowel for the tower is half the length of the long members used for the base. in my case, that makes them 6 inches.

i placed the tower dowels 4 inches from the end and put screws up into it from underneath.

the support pieces (that make the triangular shape) are simply paint stirrers that i placed where i wanted them, marked where to cut, then used a set of wire cutters to cut through them to shape. then i put them on using staples. these supports are very important, since they transfer the torque put on the tower dowels by the catapult to the base.

i drilled and screwed screws into the tops of the tower dowels. the tower dowels can be spread apart easily (which helps when installing a new rubber band). that's ok, because the only force applied to the pipe (the rubber band) is orthogonal to the pipe (a 90 degree pull - that is, it pulls away from the pipe - not along its length).

next, i simply held the tower dowels apart and inserted the pipe -- the screws hold it in place. the hot glue gun glue keeps the pipe from wiggling too much, but doesn't secure the pipe.

Step 4: Install the Ammo Bucket and Rubber Band

the ammo bucket is a tiny plastic cup that sugar free drink mixes are packaged in. (like crystal light).
i used one of the screws to poke 2 holes into the bottom of the bucket about 1/2 inch apart.

then i threaded a zip tie through those holes and around the end of the catapult arm and tightened it.

next, i spread apart the tower dowels and slid on a rubber band onto the pipe. the other side of the rubber band goes around the catapult arm and is tightened in place using a zip tie. the closer this is attached to the ammo bucket, the more tension applied to the rubber band - which causes the ammo to fly farther. if you want to experiment with different tension, then double wrap the zip tie through the rubber band and around the arm but do not tighten it fully. this provides enough snug friction to keep it in place whend using it, while at the same time allows you to adjust the tension up or down by sliding the zip tie along the arm.

you might think that because of the acute angle of the arm to the ground that the ammo will fly high and land short, but this is not the case. a ping pong ball flies well over 6 feet and can knock down a small lincoln log house, a cotton ball flies about 3 feet, and a paper wad flies about 4 feet.

This is my first instructable... enjoy!