Alan Davies attempts to answer the proverbial question: how long is a piece of string? (from here, it looks about a foot-and-a-half. The end) But what appears to be a simple task soon turns into a mind-bending voyage of discovery where nothing is as it seems. An encounter with leading mathematician Marcus du Sautoy reveals that Alan's short length of string may in fact be infinitely long. When Alan attempts to measure his string at the atomic scale, events take an even stranger turn. Not only do objects appear in many places at once, but reality itself seems to be an illusion. Ultimately, Alan finds that measuring his piece of string could - in theory at least - create a black hole, bringing about the end of the world. Fantastic. Another event which could probably cause the world to implode would be a collaboration between Annie Lennox and David Gray, but that won't stop Jools showing it tonight at 11.20pm on BBC2.