All I used for this was:
- Old skateboard deck
-One set of wheels and trucks
-Empty can of V8 soup (or some other small can that the wheels fit into)
-Nails and a board
That was all I used to make the basic balance board (that no one can balance on).
Now for the trick part I used:
-A long nail
-A small piece of clear plastic
-Two scraps of wood
Basically I just built the "impossible balance board" by taking the old skateboard deck and attaching the trucks and wheels so that the wheels were in the middle of the skateboard. Then I sawed the can in half and nailed it to the smaller piece of wood. I then wedged the wheels into the can. After the board was built I basically zip tied a nail to two pieces of wood so that the wood could rotate around the nail (see pictures) and hot glued the plastic on and then the whole thing onto the main board.
To operate the board I flick the plastic leg down to allow other people to try to balance, then when it is my turn, I discreetly flick it back up with my foot. I then step carefully onto the board (starting on the side without the plastic) and lean gently into the leg. I often try to wiggle the board around a bit to enhance the illusion. This part takes a little practice, and I would recommend trying it a couple of times before doing it for real.
I was reluctent to make a full instructable on this project because I used a lot of random parts and harder-to-find pieces, and I was thinking that a picture instructable would probably suffice to convey the basic concept and the way that I went about building it. I am assuming that anyone that would like to build this would be able to from the pictures and descriptions, but if you would prefer me to make a more complete step by step instructable on this project please let me know in the comments below! And good luck building!