A lot of the fun of an A-to-Z is surprise. People will suggest topics that I wouldn’t have considered. Sometimes I’ll do a bit of preliminary research and find that a topic is more interesting than I guessed. Unit Fractions, from the 2018 A-to-Z, is one of those. A unit fraction looks, from the definition, to be too dull to bother with: what’s interesting about one divided by a whole number? A great deal, it turns out, and as I started writing I threatened to keep on writing.