Answer: Live Frogs
Modern science has shown that more than a few folk remedies and practices actually have scientific merit and an observable mechanism in play, but one thing that modern science gives us cannot tell is exactly what motivated people to start with the practice in the first place. A curious example of this is the old Russian practice of dropping a living Russian brown frog into a container of milk to prevent the milk from spoiling.
Science can now tell us why the trick works because the frog releases a slimy secretion of its skin that has fantastic antibacterial and antifungal properties that kill bacteria in the milk. Microscopic analysis of the process aside, that certainly does not tell us exactly how Russian farmers found the method to use frogs as milk preservatives.
Fortunately, however, knowledge of practice lived on in Russian folklore because the team of researchers who discovered the link between the frogs and the pristine milk discovered 76 unique peptides in the milky secretions of the skin of the frog that potential for pharmaceutical use.