after finding how to make a bow and arrow out of office suplies here, i came up with an idea to build a crossbow, and it works great!! sorry if the pics arent that great(please dont shoot it at someone and get me in trouble)
p.s. this is my 1st instructable

Step 1: Materials That I Used

1.rubber band (the stronger the better)
2. hollowed-out pen
3. krazy glue and tape (glue isnt needed, but it helps)
4. razor or scissors
5. clothing pin
6.dental flosser (not floss itself, but the handled flosser)
Ok, so the last 2 arent "office supplies", but they still are common household items, and you can always improvise.

Step 2: Clothing Pin Stock

first, take apart the clothing pin and glue the 2 peices longways, so that the angled parts connect and form a rectangle. ( you should use the krazy glue, and NOT the tape on this step, as the tape would allow the 2 peices to slip, unless you tape it very tight and secure)

Step 3: Rubber Band Bowstring

cut the rubber band so it can stretch it out longways, and insert it through the hollowed out pen. then tie the ends together so half the rubber band is inside the pen tube.

Step 4: Attach Stock and Bow

insert the clothing pin between the band and the pen tube so the first notch of the clothing pin rests on the tube. then tape (and, yes, use tape, not glue this time, its really hard to work with in this area (at least it was for me) but it might hold better if you can manage to glue it)

Step 5: Guide

heres where the flosser comes in. cut off the handle, then glue AND tape it (as this adds durability) to the front. the purpose of this is to hold whatever your shooting out of the bow in place.... no fumbling around like with a regular bow and arrow.

Step 6: Enjoy Your Crossbow!

i made mine with really strong elastic, and it performs very well. it shoots the arrow pictured in the crossbow about 30 ft, and it maintains a pretty straight trajectory for about 15 ft.
oh, and if you find another arrow design that works well with this, please post a link to it, thanks :)