In the Xenophobe's guide to the English, page 54, under the heading Sense of Humour, the authors, Antony Miall and David Milsted, state that:
English humour is as much about recognition as it is about their ability to laugh at themselves, for example: 'I thought my mother was a rotten cook, but at least her gravy used to move about a bit.'
I am not certain I get the joke. Does 'move about' have two 'opposite' meanings on which the joke plays?
Googling the joke, I got a reference to another book, Smoking in Bed: Conversations with Bruce Robinson, which reads:
Growing up through the radio age you had Ray Galton and Alan Simpson's brilliant dialogues for Tony Hancock. Lines like 'I thought my mother was a rotten cook, but at least her gravy used to move about' are indelible in my mind.