As an author, it's not uncommon to get emails or letters correcting something in one of my books. Sometimes these corrections are useful, at other times, the correspondent misses the point. But I recently had one from Ronja Denzler that was not only correct, but also highlighted something really interesting about mnemonics. These phrases to remember something can be genuinely handy - most of us can still recall those for rainbow colours or planets (often still incorporating Pluto) from school, while I distinctly remember a woman called Ivy Watts from my physics class. But the most elegant are the numerical mnemonics, where the numbers of letters in each word represents a digit. This form reaches its zenith in the mnemonics for pi - so much so that the art of producing these has its own, distinctly tortured, name of 'piphology'. When I wrote Introducing Infinity - a graphic guide  in collaboration with the excellent illustrator Oliver Pugh, I asked if he could use a fuller v...