I first came across this idea when I stumbled upon this site: http://www.thinkgeek.com/geektoys/cubegoodies/6758/
But I didn't feel like paying for something that I could just as easily make. I first tried this using a cardboard box as my base and it worked, but the box eventually tore. So now here I am to remake a better base and to show you how you can get more use out of those magnetic darts.
The materials you'll need are:
1. A Magnetic Dart set
2. A flat wooden board (8" is long enough)
3. A 3/32" and a 9/32" drill bit
4. A Drill of some sort
5. A wire coat hanger
6. Twine (string, thread, fishing line, etc.)
8. A marker (optional)
9. Tape measurer
There is now a movie posted above the comments (it also can be found at the last step). The point behind this whole instructable (I feel obligated to mention this now) is to make a toy that both the young and old can enjoy. You move the magnetic fields (either repel or attract) around by deciding where you want each of the magnets, then let the pendulum swing to find out how the magnetic fields you've just created will cause the overhead magnet to react.
You could have just made a field to make it stand still which will be caused either by charges that are alike to the pendulum in a circle forcing it to stay between them or a magnet with an opposite charge of the pendulum that will attract the pendulum to it. You also could have made it do a dance around the base by placing magnets of opposite charges near each other and spread out making the pendulum swing randomly through the invisible fields.
Make one of these for a child to teach them about how magnets repel and attract each other (I suggest using a better board than I did and paint it). Could even be used as a science fair project if you feel like recording the locations of where each type of magnet was and how the pendulum reacted to each trial.
Forgot the all mighty WARNING label.
Warning: Scissors are sharp. Twine can tangle and choke. Small objects found within this instructable are not suitable for eating, so please do not give to small children nor shall you eat anything found within this instructable.
I will not be held responsible for the actions of others.
Note: There is now an improved paint picture below for making this ROMP. Some credit must be given to Stormed (see comments below).