Minecraft wiki entry for Transmission, associated text: Water streams can carry information up and down. For a slow downward signal, a dispenser can place and pick up a water block (or even a lava block), and an observer can detect the changes in the flow downstream. For upward signals, a column of water source blocks can have the solid block below it swapped out by pistons; switching to a magma or soul sand block instantly propagates a bubble column up the water column, while switching to a normal block removes the bubbles; an observer watching one of the water blocks detects these changes.