It came down on a Tuesday in December at about four o'clock, when the light goes early and the whole town is in a bad mood about it. Twenty-nine children were on the yard because Miss Alder had forty minutes of arithmetic left and would not shorten them, and they all saw it, and all of them shouted, and for about nine seconds the entire population of this school was pointing at the same thing over the top of a wall.

Then it went behind the wall, which was the wall of the old coal store that had not been opened since 1998, and thirty children did not go after it, because they had been in the yard long enough to know that going after something is a rule for other people.

Miss Alder came out, and looked at the wall, and said, "Children, the sky is not a thing you can put your hand in." And then she went and got the keys, because she had been teaching thirty years and she knew what a wall looks like when something has come over it. The star was in the coal store, in three inches of old coal dust, and it was about the size of a dinner plate, and it was not a stone and it was not a lump of metal and it did not have a hole in it like a bead. It was warm.

That was the first thing, and every single one of them put a hand on it before anyone said a word about anything else. It was warm the way a hand is warm, and it was smooth, and it had a shallow notch in it at the top that nobody could account for. "It is not shining much," said Kerensa, who was disappointed. "It is the middle of the afternoon," said Miss Alder. "In December. Sit down."

The class had a meeting about it, because Miss Alder had told them to, and she chaired it, and it lasted one whole lunch hour and produced a set of rules. Rule one: the star belongs to the class. Not to a person. To the class. That was Nadia's motion, and it was the only one anybody argued about, and it was carried.

Rule two: nobody tells their family a true thing about it. Not because it was a secret, Nadia explained, but because if their families knew, their families would want to hold it, and holding it is what happened to Aunt Rosamund, and rule three was therefore that nobody touches it except with two fingers, and everybody in turn, in writing, from the top of the list. There were twenty-nine names on the list. The last one was Miss Alder. "Also," said Nadia, "we should work out what it is, and we shouldn't ask anybody who isn't in this room, because then it stops being ours." "That," said Miss Alder, "is the first sensible sentence anyone has said to me about anything since 1998."

What it was, as it turned out, took them a term and a half. They weighed it, on the science balance, and it weighed one pound and four ounces, and it weighed the same in January as in November, which was itself a finding that got a whole wall chart. They tried keys, coins, magnets, the steel ruler, the metal detector Miss Alder borrowed from the caretaker, and finally a school shoe, none of which did anything at all.

They tried to scratch it, which was against rule three and which they all did once, and the nail went across it and left no mark, and nobody said anything for about a minute. Then they gave up on what it was, and Nadia wrote at the top of the wall chart, in the biggest letters she had: WHAT IS IT, and under it, in a smaller hand, WHY DID IT COME HERE, and Miss Alder looked at that for a long time and said, "That is two questions. You are allowed two. Choose well."

They built a shelter for it out of the frames from the old picture house. The picture house had gone in the summer and the frames were in the corridor, and thirty children took them apart, and thirty children put them back together as a box, and the box had a lid that opened, and glass in four sides, and it sat on the low wall at the end of the yard where the sun got to it from about two o'clock on.

The building took nine days and it went wrong four times. On the fourth attempt it would not stand square, and a boy called Ade said, out of nowhere, that it was because they were all looking at it instead of at the wall, and every single one of them turned around and looked at the wall, and the box went in straight and stayed straight. Nobody ever established where Ade got it. He said, when asked, that it just came out.

It got a better winter than they did. They had a shift rota for the two-fifteen sunlight, because that was the only time the glass box did anything worth looking at, and the rota went round the same way for months, and by February a boy who had never been in any other class at all could tell you it was warmer on the cloudy days, which was the opposite of what anyone expected. And it was never a wish. That is worth saying, because a thing in a glass box in a school yard is the sort of thing that becomes a wish by itself.

Somebody wrote WISH on the side of the box in chalk in the first week, and Nadia rubbed it out, and wrote UNDERSTOOD in its place, and everybody laughed, and it stayed up until the rain took it in March. What it did instead was shine. From the second week of January it began to give off a small amount of light of its own, and you could see it from the far end of the yard on a dark afternoon, and not much further than that, and no further than that however hard anybody looked. "We are not going to be able to measure that," said Ade. "No," said Miss Alder. "We are not."

They had it catalogued by the summer term. It is a five-page document. It has the weight in it and the date and the two of the six times in a fortnight when the temperature was taken off the lid with a clean thumb. It has the shape, drawn eleven times by eleven different people. It has the notch, and a note that says the notch is not a chip, because when Nadia said so in the corridor on the Monday, four of them immediately went and pressed their thumbs into it to be sure, and it was not a chip.

Page four is the one that mattered and it is one line. It says: it is not broken. It came all the way down and it is not broken. Nobody in the class could think of a way to write down why that was the important thing, and so page four stayed one line, and Miss Alder, who had been in the school thirty years and had taught in three of them, signed it at the bottom and dated it and did not add anything.

The log went on the shelf in the box, and the box went on the wall in the yard, and the wall was repainted round it in July without anybody arguing about it, and by September there were twenty-six children in Miss Alder's room instead of twenty-nine, and a new girl called Sunila who had to be added to the bottom of the rota and did not know why her name was last.

On the frame of the lid, where it would not weather, somebody had begun a chalk tally in the last week of the summer, one mark for every week the box had stood on the wall, and by the second week of the new school year there were thirty-eight of them, and the mark being put on that Friday was a different shape from the rest, and made at a different angle, and by somebody who had never seen the other ones done.

Behind the glass, in the last of the afternoon light, the star sat on its three inches of coal dust and gave off its small amount of light, and had done nothing at all that anybody could put a finger on, and had come all the way down on purpose.