The chances are, dear Aperiodical reader, that you’re familiar with the following chain of emotions while browsing Facebook:

- Oo! A notification!
- Oo! A message from Aunty Jean, I’ve not heard from them in ages!
- Oh. It’s one of
*those*.

- Oo! A notification!
- Oo! A message from Aunty Jean, I’ve not heard from them in ages!
- Oh. It’s one of
*those*.

The chances are, dear Aperiodical reader, that you’re familiar with the following chain of emotions while browsing Facebook:

*Pythagorean triples have a long and storied tradition. But what about the near misses?*

You’d be surprised how much math[s] you can learn by exploring some of the implications and ramifications of what may seem at first no more than a trivial brainteaser

Martin Gardner

Long before Catriona Agg’s *Geometry Puzzles in Felt Tip Pen* were all over Twitter, Ed Southall was doing something similar without felt tips. He and Vincent Pantaloni served up a smorgasbord of *Geometry Snacks* in 2017 and *More Geometry Snacks* the following year — but these are aimed at a (chronologically) grown-up market.

*Geometry Juniors*, as it says on the tin, is aimed at a younger audience. Or rather, it’s aimed at parents or carers of a younger audience; it’s as much for starting conversations about geometry as it is for direct instruction or to bamboozle puzzle-solvers.

In fact, St Andrews offered a French for Scientists course, so I ended up doing Maths with French. A win all round.

I can pinpoint the exact moment it became clear I would study maths at university. Parents’ evening, year 12, I mentioned to my French teacher that I was thinking about a French degree. He looked at me as if I was stupid and said something like “you’re good at French, but you’re *GOOD* at maths. Besides, a French degree isn’t much use.” Alright, fine. Maths it is. He was spot-on. I never looked back.

Back in the olden days, Colin entered a proof without words in the Big Mathoff. It was mentioned, in passing, in a New York Times obituary of John Horton Conway.

A book about mental arithmetic? By Rob Eastaway? Count me in! In my fuzzy mental Euler diagram of topics and authors, *Maths On The Back Of An Envelope* lies in the intersection of several ‘favourite’ circles.

Perhaps paradoxically, this meant I was expecting to be a little disappointed: how can a book, by an author I admire, on a topic I both love and have Strong Opinions about, live up to what I’d like it to be? Luckily, Eastaway’s writing is excellent, even taking into account that you expect it to be excellent.

I have two simple rules for deciding whether a popular maths book is Any Good. Firstly: does it teach me something I didn’t know? And secondly: does it entertain me when treading ground I’m familiar with?