But according to Quantum Mechanics, we cannot simultaneously know the position and momentum of the string, so we cannot both measure it and know its speed. As Mr Johnson says, 'even if it were possible to know the length of the string precisely, it would probably be moving like a bat out of hell' and you couldn't catch it to use it anyway.
Michael Rubinstein thinks it's not so difficult to compare the piece of string with a piece of elastic, while Jill Bell suggests doubling the half-length of the string.
Meanwhile, Dr Duncan Conway of the University of Hertfordshire takes issue with Prof Treble's theory of queueing, casting doubt on his assumption of rational behaviour by shoppers. 'When approaching a supermarket checkout,' he maintains, 'shoppers use whatever heuristic rule-of-thumb they have personally developed. Unless it goes seriously wrong, we do not even notice whether a queue is as fast as any other. But we do notice when it is the slowest. Our reaction then is to try to switch to another faster queue. This is why the slowest queue becomes the shortest.'
This week's queries: What is the silly question to which the answer is 'St John's Wood'? (J Arundale); Who does Gordon Bennett call upon when he's mad? (Elaine Lyne); and Kate Tadman wants to know how and why her black and white cat sheds white hairs on her navy duvet cover, while reserving its black ones for her white one. Answers and more questions to the usual address.Reuse content