Minecraft wiki entry for Clock, associated text: To make this design, place a sticky piston facing up with a redstone wire next to it on one edge. Next to the redstone wire but still 1 block away from the piston, place a solid block and place redstone wire on top of it. Then, next to that block, but still 1 block away from the piston, place obsidian two blocks up with a redstone wire on top of it. Above the sticky piston place a slime block. Finally, on top of that, place a redstone block. The clock activates immediately. It works on the principle of quasi-connectivity, and the wire directly next to the piston is used to update it.