Family conversation VI

If you have a five-litre jug and a three-litre jug, how do you get four litres? Or, if you take one set of parents and a 20-something who's returned to the nest, how do you get a book out of it?

T: "It's very fashionable to write books about one's parents these days. Everybody is doing it."
Pim: "I know, you told me the other day."
T: "I just thought I'd mention it again."
Pim: "If you wrote one about us, no one would read it; it would be boring."
T: "No, it wouldn't; it would be hilarious."
Pim: "I suppose you could put a different jug problem in italics at the beginning of each chapter."
Mim: "Don't encourage her!"