DISCLAIMER: THIS MAY BE POTENTIALLY CONSIDERED A WEAPON, AND IF YOU TRY TO DO SOMETHING REALLY, REALLY STUPID WITH THIS, LIKE BLINDING YOUR FRIENDS OR TRYING TO CIRCUMCISE THEM, IT'S NOT MY BAD, OK?

Ok, now the fun things.
Are you a Call of Duty fan and/or an office worker? If so, here is it. The office ballistic knife.
I'll make four steps: the Launcher, the Knives, the Loading and Firing, and the PSs.
Also go constructive with the comments.

Step 1: Launcher

You'll need:
a Pilot G2 pen

1)Disassemble the pen (note: to take off the spring, you can use a toothpick)
2)Saw off the pen tip
3)Insert in the back of the body the pieces in this order: Button, Spring, Piston
4)Screw back the top of the body

Step 2: Knives

You'll need:

some straws (just steal them from the canteen when no one's looking) (also, use the thin ones,not the McDonald's like)
some duct tape (usually found in the storage room)
some pushpins (one per straw)

1)Tape the pin to the straw
2)Align to the pen and cut as seen in the photo

Step 3: Loading and Shooting

Ok, this is not so good, but for shooting:

1)Insert the knife
2)Press until you hear a "click" sound
3)Press the button!

Step 4: PSs

1- Aim slightly above your target
2- Move your arm fastly forward and stop fastly when you press the button
3- If you work in a hospital or similar, keep a straw excess to make a little shooter, like the one in the picture. Of course, you could also buy one at your local chemist's, but he might get suspectful if you ask him alot of these...