I learned about this on an infomercial-or I could have been dreaming, I really don't know. Anyway, this is a fun project. I make them a lot.

Step 1: Materials

You will need:
- two toilet paper tubes
-four paperclips about the same size
-two rubber bands about the same size
-duct tape
-scissors

Step 2: Making the Inner Tube

As you will probably notice, the tubes won't fit together. We will fix that. Take one of the tubes and cut a line down it's length. Then curl it back together with one end overlapping the other a little bit Finally, Put 1 to 2 layers of duct tape around it.

Step 3: Attaching the Paper Clips and Rubber Bands

Loop a rubber band around two paper clips. The paper clips should be at opposite ends of the rubber band. Make a second one. Consult the picture if you need more detail.

Step 4: Attaching the Whole Thing Together and Firing.

Clip two paper clips onto the top of the larger tube so that the rubber bands are on the inside. Clip the other two on the inside of the smaller tube again, if you need more detail, consult the picture.
Once you've completed this step, pop a ping pong ball in the front, pull back on the smaller tube and let go.