For my first instructable, I give you the Bic Pen Cannon!

Step 1: Preparing the Bottle

Materials:
Water Bottle (any kind will do)
Bic Ultra Round Stic Grip (most kinds of pens will work)
A tool to cut through the bottom of the bottle (drills work best)
Aerosol Hairspray
Safety Lighter

To prepare the bottle, all you need to do is drill a hole through the center of the bottlecap (this may need to be adjusted in the next step) and then drill out a circle in the bottom of the bottle so you can fit a safety lighter head through it very snugly. It's very important that the lighter fits snugly, if it doesn't, there's the possibility of fire shooting out the back and burning you.

Step 2: Preparing the Barrel

Take apart the Bic pen, slide off the rubber grip, and there will be an almost arrow-shaped piece of plastic holding the ink well and pen tip. Slide that piece of plastic off while being careful not to pull the pen tip out of the ink well, making a very annoying mess, and then slide that piece through the hole in the bottle cap pointy side holding the barrel in from the inside of the cap. You may need to adjust the size of the hole if you didn't drill precisely. Then screw the bottle cap back onto the botle and you're ready to fire!

Step 3: Firing!

Unscrew the bottle cap and spray some hairspray into the bottle. Screw the cap back on, put the ink well part of the pen into the plastic barrel and push to make sure it's secure. Stick a safety lighter through the back and fire away. It should make a pretty cool high-pitched rocket-like noise and the pen cap should shoot out about 30-50 feet depending on how well your cannon is built, how much hairspray you used, etc.
I'll try to put a video of this thing firing on here soon.