noun, feminineplural: pruebas del algodón
acid test
Literally, “cotton test”. The phrase comes from a detergent TV ad from the 1980shttps://www.youtube.com/watch?v=nsnIpoZVA_8 in which a butler passes a piece of cotton across a surface to prove that it is perfectly clean. At the end of the advert, the butler triumphantly proclaims "the cotton doesn't deceive".