Farthing and 1p - grandfather and granddaughter

Scraping around for a blip today, I discovered that a 1p coin and a farthing (1/4 of an old penny before Britain went metric) are identical in diameter, though the farthing is a little thinner. I don't think I ever knew that. I do remember, when an impoverished child, discovering that a farthing worked in a 6d slot machine - except that farthings were at the time no longer legal tender then and in very short supply! But now, if I was a total idiot, I reckon I could use one in place of 1p in the supermarket self-service checkout. Wow!!!!!