This instructable will show you how to make a great pocket sized puzzle. Great for a gift.
P.s. This was not my idea credit goes to This Site . However the pictures are my own.

Step 1: Materials and Tools

To make the Bolt puzzle you will need:

A left-handed bolt (For size, look at the picture) and matching nut.
(By the way you can also use a right-handed bolt if you don't have any left-handed bolts, I have used a right-handed bolt and not had any one guess it thus far. However the puzzle will no doubt work better with a left-handed bolt.)
A split washer (Notice that I split it more, see picture).

Tools:
A file for metal.
A saw for metal.
Pliers.
A welder.
A vise.

Optional:
Drill with wire brush attachment.

Step 2: Step 1

Now take your bolt and put it in the vise. Then file out a v-shaped notch on two of the bolts sides(See picture 2).
Note: These notches do nothing, But most people will think they are the key to solving the puzzle.

Step 3: Step 2

That done cut off a piece of the bolt, about a quarter of an inch of the end of the bolt. (See picture for help).

Step 4: Step 3

Now screw the small piece of the bolt about a quarter of the way into the nut (see picture). Make sure you don't screw to far in otherwise the other piece of the bolt will not fit in nicely. Next weld them together just like in the picture.

After welding you will notice that there is a fair bit of heat discoloration on the the metal. So using the wire brush attachment on your drill buff the bolt so it is nice and shiny or you can use aluminium spray paint.

For those who don't have a welder here is some great ideas:

If you don't have a welder you can use a blowtorch (or something similar to heat up the nut and piece cut from the bolt) and some solder.
(Thanks kasssa)

Or You could also use a dab of JB Weld to secure the nut and piece of bolt.
(Thanks CimarronWarrior)

Step 5: Step 4

Now all you have to do is put it together. Look at the picture for help.
Great, you're done, Now all you to do is choose your "victim" and ask him to remove the washer without bending it. He will most likely struggle with it for about 5/10 minutes before giving up. Make sure you keep telling them that it is possible and is pretty simple, otherwise they give up sooner. When he gives up put it behind you back and remove the washer. Then put it back together without the washer. Give this back to him and if he still can't guess, go ahead and show him. He deserves it for the time you wasted.

Troubleshooting:
Wiggly bolt<>Sometimes the bolt doesn't screw tightly into the nut. So it gives the trick away,so what you can do is give the end of the bolt that's going into the nut(The nut that is welded to the small piece of bolt) a couple taps with a hammer this makes it harder for the bolt to be unscrewed. Careful to much and you'll need a pair of pliers to unscrew it, which kind of ruins the puzzle.
P.s. If you see any problems post a comment about so I can fix them.
Cheers,
Mattrox