In any case, I'm more interested in finding explanatory metaphors to help people use the tools that are currently widely available and supported than I am in...doing whatever it is to people's heads that the BDD crowd think they are doing. Anyway, I've found that it's a bit helpful to talk about test-first tests being gauges (as I've mentioned in passing before. Trouble is that too few people these days have done any metalwork.
A Metaphor Too Far
So, the important thing about a plug gauge or such is that it isn't, in the usual sense, a measuring tool. It gives a binary result, the work piece is correctly sized to within a certain tolerance or it isn't. This makes, for example, turning a bushing to a certain outside diameter a much quicker operation than it would be if the machinist had to get out the vernier micrometer and actually measure the diameter after each episode of turning and compare that with the dimensioned drawing that specifies the part. Instead, they get (or assemble, or make) a gauge that will tell whether or not a test article conforms to the drawing, and use that.
And this is exactly what we do with tests: rather than compare the software we build against the requirement after each development episode, we build a test that will tell us if the requirement is being conformed to. But so few people these days have spent much time in front of a lathe that this doesn't really fly.
But, flying home from a client visit today my eye was caught by one of those cage-like affairs into which you dunk your cabin baggage (or not). It would be far too slow for the check-in staff to get out a tape measure, measure your bag, and compare the measurements with the permitted limits. So instead, they have a gauge. From now on (until I find a better one), that's my explanatory metaphor. Hope it works.