So what is it?
The humble pen is in fact a wondrous thing. A pen is best thought of as a vehicle to deliver ink or other such pigments to a writing or other surface. This is typically done through a reservoir of ink in either a central vessel or within the body of the pen itself, and a nib (end) of the pen to regulate the flow through to the writing surface.

Origins
The origin of the pen is a natural evolution of the old stylus used to press shapes in to clay tablets. With a limited supply of suitable clay, other methods were used - chisels to drive sharp, durable styli (sp?) in to stone, and eventually the pigmentation of whatever was laying around - dead animal skin (hide), your cave wall, or any other suitable surface.

However as with most implements that have been around for a long time, there are many variations on this common central base. The most common pen these days is the ballpoint pen. Due to its ubiquitous modern nature, we shall concentrate on this wondrous device.

The Ballpoint Pen
Mass-manufactured, they are almost ubiquitous in their presence - home, work, boats, rockets, unmanned spy drones, operating theatres - all usually sport a ballpoint pen of some description. They are commonly made of 4 components - a barrel of plastic with a screw thread at both ends. In to the barrel fits a refill plastic dispenser filled with ink and an integrated ballpoint for regulating a small flow of ink to the writing surface - an internal ball is slightly smaller than the surrounding casing, allowing a dribble of ink to flow at a regular rate. The ink is viscous enough not to run everywhere (most of the time) at normal tempuratures. The refill is shaped like a straw and is thin enough that capillary action draws a constant supply of ink to the tip. The other elements of a ballpoint are a guiding nib to hold the ballpoint at the centre of the barrel; and an endpiece, to stop the otherwise unsecured refill from being pushed out the end of the barrel. Both of these are screw threaded to attach to the barrel.

And...?
Also available are fountain pens with a reservoir of liquid ink in the one-piece barrel, with an elaborate nib attached that separates with pressure to slowly dribble out the ink.

So...?
So what is so great about the pen? Well, it can be used for so much more than just write with. The barrel of a ballpoint pen alone can be used for so many things!

Weaponry
Try wadding some paper up, chewing and moistening it in your mouth, and then lodging the "ball" in the end of the barrel. Pushed a little further in, you can build up a huge blast of air with a puff of your cheeks and propel the "spitball" at huge speeds ofver vast distances!* With a little practice, you can develop amazing accuracy and performance for something that costs less than a dollar!

Medical safety
Someone choking? Can't dislodge it with the Heimlich maneuver? Caught in the upper trachea? Everyone should know how to perform your own cricothyroidotomy. It may save someones life!

Groin grabbing-ly good showpiece
The long, stable nature of all pens allow them to be twirled around a finger or wrist with suitable practice. Mathematics students everywhere have seen those talented chums spinning their pen over and around their fingers. Websites have been established. Women have been wooed. Money has been won. All from the humble pen!

MORE Weaponry!
A pen is essentially a small sword. Think about it!

Collectors items
Antique and rare pens are real value items, especially in online trading markets like Ebay. The pen Adolf Hitler used would be worth a fortune, if unachievable, whereas a limited edition Mont Blanc can appreciate monetary value in the space of six short months!

STILL MORE Weaponry!
DO NOT TRY THIS AT HOME. This stunt conducted by untrained unprofessionals
Get together a number of metallic barrels. Although harder to find, they withstand the devastating temperatures unleashed... Tape the barrels together. With a number of accomplices, gather sparklers together and arrange one inside each barrel, with the "business ends" of the sparklers brought together to a point. Take a lid from a tin can, shape it to look like a bowl. Drill a hole big enough for ONE sparkler, and fit the end securely attached to the numerous barrels. Stick one sparkler through for a fuse. Light the fuse, drop, run.

Body art
Got a girl/boyfriend? Want to live forever with the memory of them? Got more passion than sense? Try this!

*WARNING - this HURTS*

With a pocket knife, cut your beloved's name in to your arm. It doesn't have to be deep, just enough to break the skin. Then, or maybe before, cut the nib off a refill and blow the thick ink out. With your newly-acquired love wounds, rub the ink as hard as you can in to the cuts. Forever will your love be visible, especially after the scars heal and you GET OUT OF HOSPITAL!


As you can see, the pen can be used not only for inscribing on paper, but it can leave your unique mark on a person choking, your fellow students, gathering hordes of fans, the neighbours letterbox, and your prison-buddies' arm!
DejaMorgana says Another way pens can be turned into weapons is by removing the top half of the pen and using the spring to fire the cartridge at people. Astonishingly long range, stupidly low accuracy. Much fun for all.