Time Twister consists of two LEGO Mindstorms bricks communicating via Bluetooth. The master brick keeps track of the time and handles the minute digits. The slave brick handles the hour digits and the second indicator.
Each digit is made up of five layers with black and white tiles layed out in different patterns. Each layer can rotate 360 degrees before it hooks to the next layer. By twisting the top layer back and forth in a specific pattern, you can arrange the layers so that the desired digit is displayed.
[Source: Tilted Twister]