Acid test
The acid test has its origins in the court drinking regulations of Elector Christian II of Saxony (*1583 in Dresden; †1611 ibid.), where it served as a symbol that a glass had been completely emptied. The empty glass was turned upside down so that no drop remained on the nail of the left thumb.
Nowadays the acid test is often used in connection with tool-free tests that can be carried out simply with the human nail. In plastics engineering, for example, the fingernail can be used to determine the nature of a material. For instance, the waxy polyethylene can easily be scratched with the fingernail, while the harder polyamide is more resistant and withstands the nail.
