In the Peter's Water Channel, the quality controller of purified water works. Ordinary river crabs, a few. They sit in a pipe through which purified water passes and react to the appearance of impurities with an increase in pulse. The change in pulse is recorded by sensors that send a signal to the controller. All is simple. Moreover, this is the most accurate way to detect impurities in water, more accurate sensors people have not yet come up with. Cancer works alternately for several years. Then they are released (pensioned) and taken to the service of others, younger. And, most interestingly, only male cancers are employed. Feminism has nothing to do with it. The fact is that females are inadequate for this job. They are more emotional, distracted by everything – on lighting, on noise, on people, on other females... And so their pulse changes not only on the appearance of impurities in the water, but on any occasion. In other words, as with people.