Step 1: Gather the Parts
Those 2 pens are really common in France (i'm French) and are both from the same brand 'Reynolds'.
1st is a ink-eraser/rewriter pen. (top one in the image) it's like 2 pens in one, White half erases, Blue half rewrites.
2nd pen in just normal ballpoint pen (nevermind the color)
ok now why these pens in particular?? because the 1st one has a little "piston" inside to separate his 2 parts and this is movable inside and makes air compression possible! tadaaaaaa! (get it?)
ok it'll be clearer later.
you also need a razor or a sharp knife and some paper.
Step 2: Preparing:
On the 1st image here you can see the two pens without the caps (don't need those)
I'm made little arrows to show you where to cut, you just need to remove the tips of every pens.
(be careful with the eraser/rewriter one because inside are 2 (a blue one and transparent one) tubes/ink reservoirs, it won't spill or anything but some ink can get on you or on your desk and it's a bummer.)
then on the 2nd picture you can see what you keep! (colors are different on mine 'cause it's from an old 'gun' of mine)
tip from the ballpoint pen = cannon
shaft from the eraser = cylinder
little piston thingie that was separating the two inks = piston
Ink tube from the ballpoint pen = piston rod
goood you just need to assemble now (yeah it is really that fast and easy to make!)
Step 3: Assembly:
put the cannon inside one end of the cylinder, thin end first, and the piston at the other end. the rod will be used to push the piston far inside and compress the air! (after some use you'll find easier to get a bigger rod (no mis-interpreting here please).
In order to get a compression you need to seal off the other half of the tubing, the cannon. and that's where the ammunition comes in: paper.
yeah plain old paper.
so take a little piece and chew. yeah yeah put it in your mouth and chew! (try to take a blank piece of paper, 'cause you don't want your boss to see your mouth blue of ink when you smile...)
once you get a small wet ball of paper, stuff it in the cannon and press hard into the bottom of the cannon using the rod/inktube, press hard because this will seal off the whole thing. this process is like the recharging of early 19th century rifles. (maybe it's 18th century i don't know...)
now you're ready! put the rod in place and aim! (cannon facing away from you)
and shoot by pressing hard and quick and Blam!! (it does make a noise like that) the little piece of paper is propelled forward like Superman on Steroids! into the hair of that beautiful 'secretaire' who was just passing by ... that same 'secretaire' who you wanted so badly to ask out but never had the courage, but hey! let's rejoice you won't have to ask her out after that because she'll say no anyways!!
so yeah don't shoot at people in general, you don't want this little squishy paper/saliva thingie to go in you eye, do you? it's dangerous and bio-hazardous.
so kids, play nice and clean up your mess afterwards!