A lego Switch blade! The rubber band pulls the blade out and when you push it back in the rubber band locks the blue things in the back up into the raised gap so the blade stays back. Just push with your thumb nail down and the blade come out faster than you can blink. The size 4 by 13 makes it perfect for your pocket. Don't ask where I got the mech. I was just messing around when I got a mech. Also a great mech for a cellphone gun if the blade is shorter so you can put ammo in it. Enjoy!
Cool. I saw this before I made my switchblade actualy. It's not a bad idea, but I made my mech because all you have to do is slam the blade in. Like you don't have to pull the button back when your putting the blade back in. BTW, yours is not the thinnest. Yours and mine are the same. There both 4 studs wide. Well techinicaly mine is 3 studs wide, but the button thing on mine sort of makes it four.