For many years I've wanted to build a digital clock out of fluid gates. The medium would be clear water; the 7-segment display would consist of transparent tubes, into which stained oil is pushed to activate a segment. I think a gate would probably be some kind of vortex. I've never tried to build it, because

- I don't know anything about fluid mechanics

- The 'exhaust' is water, so I could only really operate this thing in the bath