We have covered these spherical ice molds before. I have used one, myself, and they work pretty well. A chunk of ice is sandwiched between two halves of a metal mold, and fast conduction of heat away from the ice, through the metal, causes it to melt wherever the mold makes contact. The mold halves slide on rods to maintain their alignment, and gravity does the rest. With a mold that starts at room temperature, it only takes a few minutes to make a nice shiny ice sphere.

This one is the work of University of Wisconsin law student Brendan O’Connor, made with tooling, technical advice, and materials from Madison hackerspace Sector67. The rods are guide rails from scrap printers. [via Hack a Day]