This is a retractable hidden blade. All parts except for rubberband, keychain ring and thread are available in the lego nxt 2.0 set. Please ignore the color of the parts. Enjoy!
Step 1: The Body
This part will be strapped onto your hand and will house the blade
Step 2: The Blade Blocker and Blade
This part will hold your blade in place till you yank your finger