YouTube user Grant Thompson of Random Weekend Projects has posted a video that shows how it's done. You need some bottles of purified water, a clock, and a freezer. When you put the bottles of purified water in the freezer, the absence of impurities such as dust or microorganisms in the liquid means ice crystals have nothing to form around, so the water can reach temperatures below freezing without solidifying.
A disturbance, though, can cause it to instantaneously freeze -- bump the bottle taking it out, and you'll have a bottle of solid ice. If you can get it that far, you can make the water freeze the instant it comes into contact with an ice crystal. It takes around two and a half hours in the freezer to get to that point, so the process is not exactly instantaneous -- but the ice is.
It sounds too crazy to be true, but you can verify it for yourself by following Thompson's instructions in the video below.
(Source: Crave Australia)