Here is the instructions to make a spring loaded blade similar to the one used by Altair in Assasins Creed out of K'nex. (I cannot take credit for the idea of creating this blade. I saw the slideshow by jollex and thought that it needed an instructable. I also fixed some of the flaws in the original design and made the blade more solid.)
You will need:
( Please not that all parts can be substituted by their metallic counterparts)

Connecters :
5 Blue
5 Orange
12 Yellow
1 green
2 white (2 rod connecter)
3 grey clips
10 Y clips
2 Elastics

Rods:
15 white
22 green

Step 1: Create the Handle

Take the 2 blue connecters. Put them so that their opening faces diagonally. Attach them by green pieces to two yellow pieces. Add 5 Y pieces where shown.

(I used metallic pieces for some. I am writing as if they were common color)

Step 2: Create the Firing Mechanism

To create the firing mechanism, build the two pictures shown. Be sure to leave the one orange "ladder" piece unconnected. It will be the locking pin and will need to pivot. It is also the trigger that will shoot your blade out.

Step 3: Create the Blade

The next step is to create the blade. I started at the tip. Get your grey 2 rod connecter and attach it on one end to a green connecter by a green rod. Attach the other end to a yellow connecter using a white rod. Now, stagger your yellow pieces, connecting them with white and green rods to eachother until you run out.

Take your blue connecter and face the opening away from the tip of the blade. This will be the part where the elastics connect to fire your blade. Connect it as you did the green piece at the other tip. Be sure to put the blue rod on the same edge of the blade that you put the 2 rod connecter on the other end. Otherwise, your blade will not shoot the point hardest. Connect the blue piece to the other 2 rod connecter via a green rod.

Step 4: Connecting the Handle and Firing Mechanism

Take the 2 pieces you constructed in the beginning and set one in front of you, 3 Y side facing away. Place a white rod in every y piece facing up. Slide the part of the firing mechanism with the 2 gray clips onto the two Y clip side, and the other parts to the other side. As you can see, the white rod connects to the Y clip, goes through the center of the orange ladder or grey clip , and connects to the corresponding Y clip on the other side.

Step 5: Insert the Blade and Attach the Elastics

Insert the blade so that the firing pin is to the front point of the blade, and the blue piece is on the same side as the firing pin.

Tie the elastics together. Inside the loop of the elastic should be the frontmost 2 white pegs on each side. The knot, if tied in the center of the 2 elastics, should go directly into the opening of the blue connecter on the blade.

Step 6: Loading and Firing Your Blade

To load your blade, pull back on the blue end of your blade until the point is almost within the handle. Your firing pin should be able to be pushed down onto the connection of the first yellow piece and the first white piece of the blade.

To fire your blade, push the tip of your trigger down until the blade releases.

Holding the handle properly is very important because the blade shoots out at incredible speed. Only hold on the top and bottom of the handle, the parts where the orange ladder pieces are. This will insure that both the blade will fire correctly and that you do not get hurt in the process.

Warning:

The blade shoots out very quickly. It can easily break things if shot close enough to them. Do not hit anyone in the head or neck as it will hurt. Anywhere else is able to be hit without injury.

Have fun assassinating people with your hidden blade.

- Supernewf54