It was a funny thing, a surprising thing, that brought Grandad back to me. It was algebra.

I collided with algebra in my first year at secondary school, and it sent me reeling. The very word itself seemed sinister, a word from black magic. Algebracadabra. Algebra messed up one of those divisions between things that help you make sense of the world and keep it tidy. Letters make words; figures make numbers. They had no business getting tangled up together. Those as and bs and xs and ys with little numbers floating next to their heads, those brackets and hooks and symbols, all trying to conceal an answer, not give you one. I'd sit there in my own little darkness watching it dawn on the faces of my classmates. Their hands would go up — “Miss! Miss!” — and mine never did. The homework reduced me to tears.

“I don’t see the point of it,” I wailed. “I don’t know what it’s for!”

Grandad, as it turned out, liked algebra, did know what it was for. But he sat opposite me and didn’t say anything for a while. Considering my problem in that careful, expressionless way of his.

Eventually he said, “Why do you do PE at school?” “What?” “PE. Why do they make you do it?”

“Because they hate us?” I suggested.

“And the other reason?”

“To keep us fit, I suppose.”

“Physically fit, yes.” He reached across the table and put the first two fingers of each hand on the sides of my head.

“There is also mental fitness, isn’t there?”

I can explain to you why algebra is useful. But that is not what algebra is really for.” He moved his fingers gently on my temples. “It’s to keep what is in here healthy. PE for the head. And the great thing is you can do it sitting down. Now, let us use theselittle puzzles here to take our brains for a jog.”

And it worked. Not that I ever enjoyed algebra. But I did come to see that it was possible to enjoy it. Grandad taught me that the alien signs and symbols of algebraic equations were not just marks on paper. They were not flat. There were threedimensional, and you could approach them from different directions, look at them from different angles, stand them on their heads. You could take them apart and putthem together in a variety of shapes, like Lego. I stopped being afraid of them.

I didn’t know it at the time, of course, but those homework sessions were a breakthrough in more ways than one. If Grandad had been living behind an invisible door, then algebra turned out to be the key that opened it and let me in. And what I found wasn’t the barren tumbleweed landscape that I’d imagined. It was not like that at all.

I’d known for a long time that he was fond of puzzles. When I was younger he used to send me letters with lots of the words replaced by pictures or numbers. They always ended 02U, which meant Love to you, because zero was ‘love’ in tennis. He was often disappointed when I couldn’t work them out. Or couldn’t be bothered to. Now I discovered that Grandad’s world was full of mirages and mazes, or mirrors and misleading signs. He was fascinated by riddles and codes and conundrums and labyrinths, by the origin of place names, by grammar, by slang, by jokes — although he never laughed at them — by anything that might mean something else. He lived in a world that was slippery, changeable, fluid.