umm... ball bearings, and bottom weighted to let them rest "upside up"?
Due to recent budget cuts and the rising cost of electricity, gas, and oil, we have decided to turn off the light at the end of the tunnel. We apologize for any inconvenience.
1981 Fiat Spider, all black... "Corvino", italian for the Raven
I just bought an enamel sticker for mine, and slapped it into my wheel caps. They've lasted surprisingly long, but are starting to show some signs of wear. I might end up getting a set of these if I had good enough tires to show them off!
It looks REALLY good on those wheels though, great touch!