Yet another quick, but interesting build from Drifty. It makes for a very nice mountainside wall. Simply press 1 button and the sandstone turns to glass panes (or anything else you put there :3) in the blink of an eye. Press the other button, and it reverts back to the sandy stuff. This can be extended up to 16 blocks (due to the 16 block piston limit.) If you replace the glass with fence gates or trapdoors, there will be a row of retractable entrances for...whatever you can think of!