Just for the cool factor, and kids young and old love pulling the string and moving the hand! So what if it isn't practical as a grabbing device for those things you want to scare or are just out of reach -- it's still cool!
Step 1: Get Everything Ready!
First, gather all the required materials. Shown here is everything you need to do this project. For the yellow forearm piece and the white base all you need to do is make sure a few key pieces are in place -- the rest of them you can create using long or short pieces, or whatever you might having laying around that will work. But we'll get to that. For now, take a look at the lego pieces for the hand as they are fairly precise after a lot of trial and error. Also note that this will require Lego Technic pieces as well as regular Lego blocks
You will also notice that this build requires 20-30 lb fishing line and .8mm stretchy cord (the kind you might use to make a beaded bracelet). These specs are important to match as closely as possible. Too thick on the fishing line and it will look gawdy and not tie correctly, too thin on the stretchy cord and your hand won't snap back into place after closing.
Step 2: Let's Build the Hand!
Probably the trickiest step to get everything lined up right. In case you missed it in Step 1, you will need a combination of Lego bricks and Lego Technic bricks. In the first photo you can see all of the fingers separated and then how to attach them to what will be the back of the hand, then how to cover up the palm of the hand and place the final Technic brick on the palm. It is vital that this gray piece be a Technic piece so it has the required holes you will need later.
The final picture of this step shows the "behind the knuckles" piece you will add behind the fingers. This piece has to match up as shown or the stretchy cord will pull the fingers back into place and they will have nothing to stop them from going back further than would look natural.
As with all of these steps, the more small, combined parts you are able to swap out for larger single pieces the better. This will add to the strength of the hand, and it will need it once you start adding cords.
Step 3: Now for the Stretchy Cord!
Cut your .8mm stretchy cord into 3 strands about 10 inches long. This will give you more than you need, but will allow you to adjust the tension and then cut off the excess cord without leaving you short handed.
In the first picture you will notice an additional piece of Lego apparatus that I simply added so I was able to attach the only "unique" piece for aesthetic value. It also helps to keep the fingers from snapping back too far.
The third photo shows the "unique" piece, but any Technic piece that has at least 3 holes can be used. This one just looked cooler. Tie a piece of stretchy cord to the index, middle, and ring finger at the top hole. I use a fisherman's knot but a basic knot should hold. Once attached to the finger tips you will run each cord through a different hole in your "unique" piece, down the end of the hand, and wrap it around to the front of the palm. The two 1x6 Technic pieces on the back of the hand toward the wrist will help direct your stretchy cord and will also help things look a bit more streamlined in the end. Once wrapped to the front, hold the cords in place together and add the last flat plate to the palm which will hold the tension and the cords in place.
Once you are done with this step you will have most of the hand portion build, and if you tug on the fingers with your hand they should close (but not all the way to a fist), and when you release them they should snap back to an open hand position. Testing it at this point will also help you determine any weak areas that need tightening up. It will also help you determine if the cord is pulled too tightly. If you struggle with your hand to pull the fingers down, it is too tight. The fingers should fold with a light amount of pressure and snap back quickly but not ferociously. This may need adjusting as you go, but if so you only need to remove the plate on the palm, pull the cords, and replace the plate.
Step 4: The Fishing Line
Adding the fishing line isn't too difficult. 15-20lb line is ideal, 30lb line would be the max I would go as it gets hard to tie knots if it is much thicker. I used a 5 hole Technic piece and stuck with the middle three holes.
You will need 3 pieces of line about 15 inches in length -- again, we will trim the excess off when we are done.
Tie the three lines into the three middle holes of the Technic piece. I used the basic fisherman's knot, but any knot should work. Once the lines are tied to the Technic piece, run them up through the gray Technic piece on the palm of the hand, and tie them to the fingertip of each corresponding finger. You will want to allow the first Technic piece you initially tied the line to have about 6 inches of slack, so once you have tied the lines to the fingertips hold the hand up and your line should dangle below the bottom of the wrist about 6 inches.
You can trying moving the fingers now with the "pull bar" we just created, but be sure to pull the line down and not out from the hand. Pulling out will likely cause your Technic piece attached to the wrist to pop off.
Step 5: Step 5: the Forearm
Two vital parts of this step need to be addressed. First, please notice in the first picture a new yellow piece I added to the back of the hand at the wrist. This is important to have or you won't be able to attach the hand to the wrist with any amount of stability. Its counterpart can be seen in the middle of the second picture (a corresponding 2x3 regular brick). These two will be combined to add the hand to the forearm.
Second, the tunnel in the middle needs to be to the dimensions shows. Remember that we have already attached out line to our fingertips, so the "pull block" will be threaded down through this tunnel. If you don't leave enough space you will end up having to either start over or make your forearm around the lines which can get a bit tricky.
Make note of the shape of the forearm. You can create this shape with any combination of Lego bricks, but trust me when I say that the more single, solid pieces you use, the stronger the integrity of the forearm. If the forearm is strong, it will make pulling your "pull bar" out of the bottom of the forearm a lot easier for you, and it will alleviate the fear that the lines are going to pull out and wreck the forearm once pulled.
Step 6: Step 6: the Base
Get creative with this part! I used a basic base and attached the forearm to the base as shown, but you can make any kind of decorative base you want as long as the opening at the bottom of the forearm remains as shown.
IMPORTANT: Before attaching to the base be sure to thread the "pull bar" down through the tunnel in the forearm and out the space at the bottom.
Step 7: Step 7: Complete!
You hand should drop down and fit perfectly into the forearm. Make sure to combine the two, yellow 2x3 pieces I mentioned earlier were vital, then attach it all to the base, and you are good to go! The "pull bar" should be accessible since we threaded it through in the last step, so give it a light tug and viola! Different color patterns can be used to add your own personal touch.
Leave this sitting around, and your friends will want to play with it. Like I said, so what if it doesn't have any super practical uses -- it's still cool!