Minecraft wiki entry for Piston, associated text: The key is that redstone power can be transmitted through solid blocks, but not transparent blocks. However, power can pass through only one solid block at a go, it cannot be passed from one solid block to another. A solid block can power wire leading from it only if it was "strongly powered" (by a redstone torch, repeater, or comparator, but not by redstone wire). Redstone dust can be placed on some transparent blocks, but then it transmits power only upward, not downward (that is, not through the transparent block).