I was playing Assassin's Creed the other day and I really wanted one of those awesome Hidden Blades, so I made one out of K'nex. It uses a low amount of pieces, it's easy to conceal and the blade locks in place when it's out, so it can be really dangerous.
Step 1: Required Pieces
2x Grey Rods
3x Bendy Grey Rods (Purple)
1x Green Rod
1x Light Grey Connector
1x Orange Connector
9x Red Connectors
5x Tan Connectors
A Rubber Band
29x Blue Spacers
You can replace 3 Blue Spacers with 1 Grey Spacer, but you need to have at least 5 Blue Spacers.
Step 2: The Base
This is the base of the Hidden Blade. It's very simple.
Step 3: The "Blade".
These are the two types of blades, but if you use a sharpened blade then it easily gets caught in the trigger and jams. You only need to build one.
Step 4: Putting It All Together.
The final instructions to getting it all together.
1: Put the Blade into the Base.
2: Attach the Rubber Band.
4: The "straps". If you don't have wrists as skinny as mine, well, I can't think of anything. You can find a way to make them wider. =P