Hi There, I've been toying around with wooden peg ideas using elastic bands to give them grip when it dawned on me. The same principle could be used to create a really simple and really small catapult.

So here it is, a step by step guide to make the worlds simplest catapult. Ive used 3D models drawn in Google Sketchup for each step as I thought it might help you guys understand each step a little better.

If you have trouble reading any of the dimensions in the pictures try downloading the instructable as they are difficult to read on mobile devices but I made sure they look fine in normal PC/Mac browsers :)

UPDATE: I have recently modified my way of working with this project. I have started holding two thinner pieces of wood together with masking tape to make the block which saves the slicing in half step. Hope this helps

What you will need:
A scrap piece of wood
1x 25mm of dowel (approx 9mm in diameter)
1x elastic band

Step 1: Marking Out

Although the actual size of the catapult is not important the proportions kind of are, so for the prototype I made I used simple sizes to illustrate the proportions as best I could.

Start of with a block of wood and mark it out as shown. Please note the diagonal line will only only shown on this step and on the step it is required for in order to save confusion.

Step 2: Drilling the Holes

When you have fully marked out your wood. You need to drill it at the two crosses. The top hole needs to be around 16mm in diameter and no more than about 6mm deep. This will act as the catapults ammo cradle (sorry I don't actually know what it is called but you get the idea). Make sure this hole does not pass half way through the wood. You will see why later on.

The hole on the side goes all of the way through to the other side and should be the same size as the dowel you will be using. Ensure the dowel is a fair size so that the mechanism works easily (as you can see from the size of the hole it is roughly half of the thickness of the piece of wood so if you are tweaking sizes bear that in mind).

Step 3: Cutting the Slots

Using a small needle file, dremel, band saw etc (whatever you have to hand and feel comfortable with) cut two slots in the wood as shown.

Step 4: Forming the Body

When you have your holes drilled and slots cut simply cut the body down the center as shown again using whatever tool you feel comfortable with (tenon saw, band saw, dremel etc).

Step 5: Add the Firing Angle

As you can see the diagonal line indicates the area that needs to be removed in order for the catapult to work. This section can be cut away, filed or sanded off. I used a disc sander to do this.

Step 6: Fitting the Hinge Mechanism

Probably the simplest mechanism but it works great. Simply glue your dowel into the channel of the bottom piece of your body as shown.

Step 7: Almost There

When the glue has dried pop the two parts back together and attach the elastic band. The tighter you make the band the stronger your catapult will be. Make sure the band goes into the groves.

I also shaped the top section of the catapult before attaching the band to make it look more realistic, however this is just an aesthetic choice and does not help the function of the catapult in any way.

This is simply the basic concept. If you plan ahead you could even add wheels and other decorations to it but I didn't want to insult your intelligence by showing you how to add embellishments.

Hope you enjoyed this instructable. Take care :)