Minecraft wiki entry for Piston, associated text: This is a ring of blocks attached to regular pistons at the corners so it can rotate. The blocks are usually a combination of solid and non-solid blocks. The pistons are often connected to a clock so that they rotate the ring. Most (if not all) rings have a reading head consisting of a repeater pointing at the ring and a redstone torch powering the repeater. By using redstone on the other side of a ring, one can see which type of block is in front of the reading head (1 = Solid; 0 = Non-solid). This information now can be passed to a circuit. By using a comparator and reading a cauldron's fill level, 3 additional memory cell states are available. For repeater-based reading heads those act as zeroes.