Water jar on the door - a possibly nasty trick

It rings. You open the door. A sympathetic middle-aged gentleman with a serious expression suggests himself - because danger may be imminent - to check your drinking water for "quality". The verification is of course free of charge. The personable gentleman was invited into the house by a neighbor. The values there were alarming and so he decided - since you are connected to the same line - to be active throughout the house. That the water quality depends on the one hand on the quality of the water supplied by the waterworks (and it is mostly flawless) and on the other hand depends on the house pipes and fittings at home, the likable gentleman is silent ... if you want to save money, then read over one frequently used trick, which the Wasserleitungsverband Northern Burgenland and AK Burgenland have now uncovered.