Since they are no longer sold AFAIK, and there are too many people lacking astrojax:

The original idea for astrojax was developed when the physics student tied hexnuts to each end of a string, then strung one nut loose on the string between them. There is nothing stopping you from doing this. It’s the same sort of fun, just slightly less padded than the foam astrojax. If you change the number of nuts strung in the middle to two, you can put a little duct tape around them to make a heavier middle weight, and can also change the number of nuts at the ends by tying them together at the end. Rather than having a nut on both ends, you can attach a keyring to one end and have a yoyo loop to work with for greater grip security.

Those were actually pretty fun.