Comment by IWeldMelons

Those who lived in USSR remembers soda vending machines (they poured your drink in a glass cup; you were expected to wash it before using by pressing on a cup, which stood upside down on plastic plate with holes, kinda inverted shower head; very unhygienic, I know). Well it had a button behind that let you have a free drink. You could also "upgrade" pure carbonated water (1 kopeyek) to a sweet soft drink (3 kopeyek) by pressing another button. needless to say schoolchildren would abuse the hell out of this "feature".