I'm contemplating whether a message could be communicated via a liquid in a tube. The minimum I can think of is an interleaving of two distinct liquids, e.g. a transparent and a dark one (here I try with espresso :). This is the first attempt ever, so many things are not well done yet. For instance, there seems to be a lot of air in the tubes, and having a distance between the valves and the pump means that when changing the valve, a lot of stuff from the just closed valve is still sucked in by the pump... Also lots of bubbling. A light sensor is still missing. The motor is probably too fast and too loud, I might try with a smaller tube diameter.