With NFC sensors and accelerometers, they can detect the presence of other cubes plus any movement you may make. The set of three cubes comes with a USB dongle that can be plugged into computers, where a free Siftrunner app runs that controls the mini games and puzzles on the cubes. Unfortunately there's no audio from the cubes, but if you crank your laptop's volume up, you'll have a soundtrack for your kid's playtime...or yours.
Several games will be included out of the box, but if you'd like to purchase more they can be downloaded from the Siftrunner program for as much as $5 each. On sale soon, the starter pack of three cubes, charger dock and dongle costs $149. [Sifteo via Werd and Ars Technica]