Logic gates from everyday materials Logic gates are an abstract device which can be implemented with electromagnetic relays, vacuum tubes, or transistors.  These implemenations have been successful in computing in part because of various properties of chainability, durability, and size beyond their basic binary stability.  They also work well because electricity is the energy source which can rather easily be shipped around.

I've seen adders built out of wood, marbles, and gravity.  I've seen "lab on a chip" capilary-action-driven prototypes.  I've seen all kinds of specialty mechanical calculators (Curta, slide rule).  I've seen domino trails as single-use logic gates.

I'm interested in other illustrative computing devices that aren't necessarily convenient, durable, or fast, but which exploit properties of everyday materials to perform computation and which are directly visible.  The dominoes trails are close, but are a little too complicated to reset.

Magneto-mechanical arrangements?  Water in pipes/troughs?  More general marble contraptions?

PS.  Here's a new one.  Mechanical CPU Clock
It is possible to build a Turing machine with "Lego" elements without any electric components, only with pneumatic transmission of energy. The guys who did this used some logic gates with pneumatic but this was the very easy part. The underlying automaton was way harder to build. (webpage in French. The other Turing machines in Legos use electronic devices which is very much like cheating)

Moreover I know you can build logic gates using only Lego elements and that may be more relevant.

So mechanics as well as pneumatics are enough to build logic gates.