The hammer rang once. A spark jumped off the anvil and did not go out. It waited. They followed.

The case the spark stopped at held something Tara recognised: a bicycle dynamo, the small metal cylinder that rubs against the tyre and lights the lamp when you pedal. This one was scratched and a little rusty. A thin pair of wires came out of it and went nowhere.

The card said: From a windmill built out of scrap by a fourteen-year-old who had been sent home from school.

This is a true story. Where nobody knows something, the story says so.

William Kamkwamba was born on 5 August 1987 in Malawi, a narrow country in southeastern Africa, and grew up in the village of Masitala, near the town of Kasungu. His family farmed maize, the grain that most Malawians eat every day, and a little tobacco to sell.

He liked to take radios apart to see how they worked. Neighbours brought him their broken ones, and he fixed them. He wanted to know where the music came from, and why a radio went quiet when its batteries died. Batteries cost money. He had noticed that a bicycle's lamp did not need batteries at all. When you pedalled, the little dynamo on the wheel made its own electricity, and the faster you went the brighter the lamp. He had once held a radio's wires against a spinning dynamo and heard it play. He did not yet know what that meant.

In 2001 there was a drought, and then floods, and then a famine. The maize harvest failed across the country. In William's village people ate one meal a day, and then less. Families sold what they owned to buy food. Some people walked for days looking for anything to eat. Some people died.

William had just started secondary school. That year his family could not pay the fees, which were about eighty dollars. He was fourteen. He was sent home.

He watched his friends go to school without him. His father, a farmer who had always worked hard, could do nothing about it; there was simply no money, and the family needed every coin for maize. William did not blame him. But he did not want to fall behind. So he went to the one place nearby that had books: a small library in a primary school in the next village, which had been stocked with donated textbooks.

The books were in English, which William could read only a little. He looked at the pictures and diagrams, and worked out the words he did not know from them.

One book was an American school textbook called Using Energy. On its cover was a row of windmills. Inside, it explained that a windmill could turn a generator, and a generator could make electricity. He remembered the bicycle lamp. A dynamo was a small generator. If the wind could turn a wheel the way his legs turned a pedal, the wind could light a lamp.

He read another book, about physics, that explained magnets and coils of wire, and why a turning magnet inside a coil makes a current flow. He read it slowly, a page at a time, with a dictionary when he could borrow one.

In 2002, only a small fraction of people in Malawi had electricity at home. In William's village nobody did. When it got dark, families lit paraffin lamps, which were smoky and expensive, or went to bed. And in the dry season the fields could not be watered, because there was no pump.

William thought: if I build a windmill, we can have light. And if we can have light, maybe one day we can have a pump, and then we will not starve when the rains fail.

He was fourteen. He began collecting scrap.

He walked to the scrapyard near the village and searched it for months. He came back with a tractor fan. He took a broken bicycle frame, which would hold the turning parts. He found an old shock absorber and pipes. He cut a plastic pipe in half lengthways, heated it over a fire until it softened and pressed it flat to make blades. He used the bicycle dynamo to turn the spinning into electricity. He built the tower from the trunks of blue gum trees.

He did not have the right tools, so he made them. He heated a nail until it glowed and used it to burn holes. He used a bent piece of metal as a wrench.

Nothing fitted the first time. The blades were the wrong angle. The bicycle wheel wobbled. The tower swayed when he climbed it. He took parts off, changed them and put them back, again and again. There was no teacher to ask and no one in the village who had ever built anything like it. When he was stuck, he went back to the library and looked at the diagrams again, and worked out one more piece.

The people in the village watched him dragging junk home and climbing the tower. Some laughed. Some called him misala, which means crazy. Some thought he was smoking something, or trying to do magic. His mother worried that he had gone mad from hunger.

His father let him keep going. So did his cousin Geoffrey and his friend Gilbert, who helped him carry, saw and climb.

One day the windmill was finished. William climbed up, connected the wires to a small bulb, and let the blades go. The wind turned them. The bulb flickered, and then it glowed.

People came running.

The first windmill gave enough power to light a few bulbs in his family's house and to run a radio. It charged neighbours' mobile phones. People started queuing at the Kamkwamba house to charge them. The same people who had called him crazy brought their phones and waited politely for their turn. William was already thinking about what the windmill could do next, and what a bigger one might do.

William kept building. He made circuit breakers out of nails, magnets and wire, so the wires would not catch fire in the thatched roof. He made a bigger windmill. Later he and his friends built a solar-powered pump, so the family could water a vegetable garden in the dry season. That was the thing he had wanted from the beginning: not just light, but never being that hungry again.

None of it was written in the books exactly. The books showed him that windmills existed and roughly how they worked. Everything else he worked out by trying, failing and trying again.

In 2006 an education official visiting the area heard about the windmill and came to see it. A newspaper in Malawi wrote about it. Then people writing on the internet, which almost nobody in William's village had ever used, began to share the story.

In 2007, when William was nineteen, he was invited to speak at a conference called TED in Arusha, Tanzania. He had never been on a plane before. He had never used a computer until shortly before the trip. On the stage, in English he was still learning, he told the audience what he had done.

He said one sentence that people still quote, in the words he had then: "I try, and I made it."

Afterwards, people came up to him in the corridors to ask how he had done it. Some of them were engineers and scientists. A boy who had been sent home from school for lack of eighty dollars was explaining a windmill to people with degrees, and they were taking notes.

People who heard the talk helped pay for him to go back to school. He went to a school in South Africa for young African leaders, and later to college in the United States, where he studied environmental science and graduated in 2014. In 2009 he wrote a book about his life with a journalist, called The Boy Who Harnessed the Wind. Ten years later it became a film.

Tara looked at the dynamo for a long time.

She thought about the library, which someone had stocked with secondhand textbooks without knowing who would read them. Some person, somewhere, had packed a box of old American schoolbooks and sent it to a village they would never see. That person had no idea that one of the books would become a windmill. She thought about the word misala, and how the villagers who said it were not cruel people. They were hungry people watching a boy waste time on junk when there was no food. She thought about the cousin and the friend, whose names are less famous, who carried the blue gum poles. And she thought about the phrase in the story that everyone remembers, which is about trying, and the part the short version leaves out, which is that he was not in school because his family could not pay eighty dollars.

She thought about how long a famine year must feel at fourteen, and how much of it he spent in a library and a scrapyard.

The spark sat on the edge of the case and waited.

If you had to teach yourself something hard with only the books in one small room, which thing would you choose to learn first?

The spark dimmed to a warm dot and settled back on the anvil. The Green was just the Green again.

Back on the Green

At the Whetstone that night, Tara wrote one line and dated it.

He was fourteen. He began collecting scrap.

Danya read it and frowned. "Who decided eighty dollars was the price of school?" he asked. Tara did not know. She wrote his question underneath.

Which line from this story would you keep?