I don't get this analogy. Apart from the fact that in your example the produce is validated using the exact same produce, you are actually describing the perfe

I don't get this analogy. Apart from the fact that in your example the produce is validated using the exact same produce, you are actually describing the perfect test: Two magnetized needles will validate each other, so both the product (needle#1) and the test-setup (needle#2) will be confirmed as valid in one step. If one is not working, the other will self-validate by pointing at the earth magnetic field instead...