While everyone in their right mind would love one of those fancy giant ice ball makers that go for $1300 and up, they're not the most practical solution for the home mixologist.
So I used my superbrain and did all sorts of math with equations and integrals and all, and came up with a way to make these at home:
(glasses by CB2, by the way)
To see my highly scientific secret method, continue reading after the jump...
I used water balloons ("water bombs" according the manufacturer) that cost five bucks for 400. Slightly cheaper than a fancy machine, and seriously, who doesn't like balloons?
Note: Ice balloons are for drink cooling and refreshment purposes only. Do not throw frozen water balloons. This may lead to serious injury.