A good example of a solution to a detection problem: Swiss scientists found a common food sweetener, acesulfame potassium, that does not disintegrate in water treatment processes. Now they can use it to trace sources of the country's water supply.

"We can determine what fraction of waste water ends up groundwater," says Buerge. His study in the lower Glatt valley of Switzerland found that between 10 and 20 per cent of water that was pumped from ground-water aquifers had made its way there from domestic waste water.

Since the water (payload) itself cannot be detected, a stable intermediary (tag) is introduced to solve the problem. ( 28/37 -> principle 24).