The idea meant nothing to Lena. Looking down at her feet, she noticed that the floor was made of wood – not like the planks of the wharf where she lived, but small pieces fitted into patterns. Kaufmann’s voice went on, unheard by her as she stared at the herringbone. She was never going to see the point of letters; but shapes and patterns … Perhaps she could understand what lay behind them.

In a break between lessons, the children were allowed into the courtyard. Lena hung back against the wall beneath a high windowsill and watched. There were groups of boys, circles of skipping girls, calling to one another, playing with a ball or with stones they picked up from the ground. Presumably they had all been meeting in secret, practising games over the years, while she had done nothing but gaze at the riverboats unloading and tell stories to herself.

Without moving her head, she allowed her eyes to run over them. She wondered whether each girl and boy might be like her underneath – with feelings – but there seemed to be no evidence of it. Their large bodies and their shouting made it seem impossible that anything could be going on inside.

Sometimes Herr Kaufmann wrote numbers on the board. He seemed to be saying that, put together, these numbers could lead to other numbers; but why this mattered he didn’t say. Lena was drawn to the look of eight, because it was shaped like two breakfast rolls, but she didn’t really care how it differed from four with its hard crossbar. At lunchtime they went into a different room, where a grey-haired woman in a pinafore, Frau Schmidt, ladled out clear soup with bits of pancake in it. Afterwards there were cooked apples with honey and walnuts. Seeing that none of the other children seemed to like the nuts, Lena took some handfuls and put them in the pocket of her skirt to eat later. A boy called Lukas began to call her ‘the Squirrel’ and other children joined in the laughter. ‘I bet she has a bushy tail as well,’ said Emma, Lukas’s sister.

Lena was pleased that someone had noticed her. She filled her cheeks with the walnuts, pretending to be a squirrel, but although Lukas and Emma and a couple of others laughed out loud there was something about the sound that wasn’t humorous. ‘Take some for your father,’ Emma said.

Once the large hand on the clock had risen to the upright and the small one lay square, pointing to what she now knew was a three, she was gone, running over the courtyard and out onto the street as the handbell was rung by Frau Schmidt. For fear of being found out, she kept running until she had crossed the river.

Coming back from school one day, Lena found her mother sitting on the wooden deck outside her room, looking over the river and smoking a cigarette.

‘I’ve had an idea,’ she said. ‘I think it’s time you met your father.’