This is a weapon that can easily be made at work and is meant for firing at boxes or paper. Please read the caption on Step 6 for safety and rules when using.

Step 1:

Overlap the 4 pencils as shown in the picture!
You will also need at least 2 tight rubber bands (I suggest 4 or 5 though.)

Step 2:

Rap the rubber bands around the 4 pencils. Making it look like a T. Tape the 2 pencils right before the erasers and also may want to tape the other 2 at the bottom (this allows for easy handling.)

Step 3:

Cut the rubber bands you have chosen for the "draw string" on the crossbow!

Step 4:

By placing the rubber bands in the pen caps the placing the pen caps over the pencils, it creates a snug fit and requires no tape or glue (this allows for easy changes in the rubber bands, so your crossbow isn't stuck with broken rubber bands on it!)

Step 5:

Tape down an office clip to the end of the main 2 pencils.
*UPDATE*
Also tape the inside of the clip (where mine is yellow) so it doesn't come up when "cocked."