Bored at work? Build your own 3" trebuchet out of paperclips and throw balls of blu-tac up to an amazing 4 feet. WOW!!
Please excuse the number of steps. It is really easy and can be made within an hour and from bits lying around the office, hence very cheaply.

Step 1: The Tools and Materials List

You will need:

1 pair of pliers (needle nose would be best, where available)
1 pair of scissors (yay for scissors)
8 paperclips
Cellotape
thin string/thread
Blu-tac
Corrugated card (approx 6"X6")
Ballast (I used a bunch of batteries from our recycling box)
Rubber band

Step 2: Straightening the Paperclips

Use the pliers to make the paperclips as straight as possible.

Step 3: Now Shape Them

At the very end of four of the paperclips, make an eyelet big enough to insert an ex paperclip through (but not much bigger).

At the other end, make a dog leg. Try to make it on the same plane/axis as the eyelet.

EDIT: Ignore the dogleg. Instead bend approximately 1/2" of the non-eyeleted end at right angles (90 degrees) to the eyelet. All will become clear very soon.

Step 4: The Axle

Next, shape the axle. Make it so that the distance between the hooked ends is about 2". It should be wider than your intended ballast. The V shape (U shape in this case. Needlenose pliers are best for this sort of thing) must be central on the axle.

Step 5: The Arm

Make an loop in another paperclip about 1/2" from the end. At the same end make another eyelet.
Make a small hook at the other end.

Step 6: The Trigger

A little hard to describe. Just look at the picture. The bump on the left is to pull. The loop on the right is there to catch the little hook on the arm.

EDIT: See the adjustment I made to the firing mechanism in the final step. It works so much better.

Step 7: The Components

Here's what you should have so far.

Step 8: Assembling the Parts

Take the axle and insert it through the loop in the arm (not the eyelet) so that it sits in the V. I wrapped Cellotape around the axle either side of the V just to give it some thickness (so it doesn't slide all over the place).

Step 9: The Base

Mark the base with a center line. The two lines are about 3" apart and the little Xs on those lines are also 3" apart.

EDIT: Cut the card to 3"X6" with the grain of the corrugation laying along the 3" distance. Mark the center line along the 6" distance and two lines 3" apart at right angles to the center line.

Step 10: The Base (cont)

Poke the dog leg ends of the supports into the four Xs so that the ends are between the layers of the corrugated card. If you made the dog legs at the right angle (not a right angle though), they should sit as nicely as mine. Maybe making the dog legs at this stage would be better?

EDIT: The new style supports from step 3 don't get pushed through the card as shown here. Instead, insert them into the raw edge of the card. You don't have to worry about the angles of the doglegs being incorrect, and these will be much easier to secure in position with Cellotape.

Step 11: Mounting the Axle

This is a little bit fiddly but poke the hook ends of the axle through the eyelets in the supports, then bend the hooks down with the pliers.

After that, tape between the legs to make an A-frame and then from there to the sides of the base to prevent side to side motion

Step 12: Mounting the Trigger

Push the end of the trigger into the layers of card where the center line intersects the support line(the side that the arm hook is on).
The other end of the trigger needs to run in a slit so that it can slide back and forth. Don't make the slit too long or else you risk the trigger being pulled out.

EDIT: See the last step for attaching the improved version.

Step 13: The Ballast

Make a little hook and tape to one of the batteries.

Use the rubber band to attach a whole bunch more batteries around the one with the hook

When they're all assembled and hooked on the arm make sure the ballast is not touching the base.

Step 14: The Ammo

Take the string, put a loop in it and push the free ends into a pea sized lump of Blu-tac.

Wrap it around a couple of times and then smoosh the Blu-tac into a ball.

The extra string inside will allow you some length experimentation to gain maximum distance when firing. A good length is about two thirds the distance from the hook to the axle along the arm.

Step 15: Finishing Up

Hook the ammo over the arm, pull the arm down and trap the hook with the trigger.
Position the ammo on the center line and at full extention of the string.

Aim and fire by pulling the loop on the trigger towards you. CAUTION!! The arm moves fast and has a sharp little hook on it. Don't use near your eyes. Also, this isn't the most acurate piece of equipment ever designed. Just because you're aiming it forward doesn't mean the Blu-tac is going that direction. Make this at your own risk. And make sure your laces are tied. And no running with scissors.

Step 16: Extras

I've already mentioned changing the string length to change the way the ammo flies. Also changing the weight of Blu-tac, changing the weight of ballast and altering the hook angle will affect the flight path of your ammo

Step 17: Release Pin Modification

I think a few of you have already worked this out, but never mentioned it. Well, I found a more reliable way of launching the ammo a while back when I was building a new one but have only just got around to documenting it.

I rotated the release pin 90 degrees so that now instead of drawing back like a bolt, it's hinged and rolls back off the hook of the arm. You launch by pulling back on the lever.

Very simple and you can use the original pin, though it may need some reshaping. Just make sure that it's placed far enough foward to release the hook and that it doesn't snag the ammo on it's journey past.