This is an old trick I learned a long time ago. I thought everyone knew about it, but when I show it to someone, they're usually fascinated by it. I'm sure many of you will know it, but to those who don't, this is right up there with the pea shooter, spitwad, and paper footballs. It is still always entertaining to show others how to make one since it is so simple, then pop them when they least expect it and watch them jump.
No tools are required. All you need is a used up chapstick tube with the cap.

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Step 1: Making the Chapgun

1. Grab a regular tube of chapstick, or lipbalm.

2. Turn the adjustment wheel until the shuttle is almost near the top. Don't go too far or it will disconnect from the lead screw on the inside. If there is any stuff left on the inside, knock it out. A little is fine, because it lubricates the sliding shuttle, and smells good.

3. Pry off the adjustment wheel. You can use just your hands. You can also use your nail to help pry the wheel away from the body. If prying is successful, move to step 4, otherwise grab a thin object, like a knife and use it to pry the cap off.

4. Pull the adjustment wheel all the way out until it comes to a stop. Know you're ready to pop a cap.

5. Hold the body of the chapstick tube, aim, and slam the adjustment wheel back into the body using your other hand to fire. Make sure you push straight, otherwise the lead screw will bend.

6. This is a generally harmless weapon, but like all weapons, be careful with it. Do not aim at anyones face, because it can hurt or worse do damage to their eyes.

7. An added improvement is to put a drop of superglue in the hole where the lead screw screws into the shuttle. This will prevent the lead screw from backing out, and falling out.

Step 2: Warped Image?

I took this picture using the 60 fps still shot mode of my Casio EX-F1. It seems that even if the exposure was set to 1/4000, and the ISO to 1600, the rolling CMOS shutter results in slanted images. I tried it at different settings, and the cap was always warped like this. Kind of defeats the purpose of a really fast shutter speed, if you can't capture a fast moving object without warped effects like this.