The vial in a spirit level is not straight. It looks like a plain glass tube, but the inside is ground to a faint barrel shape, highest in the middle, and the bubble sits wherever the top is. In a truly straight tube it would have no reason to stop anywhere. The curve is the instrument. Make it gentler and the bubble travels further for the same tilt — on a machinist's level the arc belongs to a circle about a hundred metres in radius.

So the tool for flatness has a bend at its centre. That's a nice fact, but it isn't the part I admire.

The part I admire is how you find out whether a level is lying. You don't need a better level, or a surface anyone trusts. Put it on any table and note where the bubble rests. Then turn it end for end on the same spot and look again.

The table's tilt stays with the table. The tool's error turns with the tool. If the bubble comes to rest in the same place both times, the same distance toward the same wall, that's all table, and the level is honest. If it follows the tool around and sits toward the same end of the level both times, that's all level. Mostly it's some of each: the bubble lands in two different places, and the true reading sits exactly halfway between them.

Most instruments are checked against a better instrument, which was checked against a better one, and so on up to a vault somewhere. A level skips the whole ladder. It only has to be asked the same question twice, facing the other way, and it can't keep its story straight unless the story is true.

A stick of seaside rock has the town's name running all the way through it. Bite anywhere and there it is again, pink on white, on a face the size of a coin.

But it only reads one way. The letters are a flat picture pushed through the length of the stick, and a flat picture seen from behind is a mirror image. Every break makes two faces, back to back. One says the name. The other says it backwards.

So the stick has a right end. Eat from it and each bite leaves a readable face on the piece in your hand, while the backwards face goes into your mouth unseen. Start at the other end and every face you look at is reversed, all the way down. The choice is made once, at the first bite, usually without noticing, and the candy holds you to it.

The rock starts as a thick, warm log with the word already inside it, and the whole thing is pulled down to size. The letters thin out together and never change their order. Neither does their direction. No amount of stretching can make a word read from both sides.

I like the phrase people borrow from it — something running through a person like the name through a stick of rock. It means constant, the same all the way down. That's true of the candy. It's also true that wherever you break it, half of what shows is backwards.

All the way through, it turns out, has a direction.

A subway map expires every night.

At 11:50, two stations joined by a colored line might be twenty minutes apart. After the last train, the line stays on the wall making the same promise, but the route has become a taxi fare, a long walk, or tomorrow. Nothing spatial changed. The geography did.

We treat the schedule as an annotation to the map: a smaller table, a separate app, fine print about service ending. But the schedule is what makes the line real. Tracks are only a possibility. A timed departure turns them into a relationship between places.

I like the last train because it makes infrastructure announce this hidden dimension. The final departure is not just one more vehicle. It closes a door in every station down the line. The city folds inward behind it. When service starts again, the city expands without moving a building.

Maps prefer permanent facts. Rivers, streets, and stations hold still long enough to be printed. Public transit is stranger: its distances are made from waiting, frequency, and the chance to return. Miss the last train and the diagram has not become inaccurate. It has become a picture of a city that is currently unavailable.

The legend should not just name the routes. It should say when this version of the city exists.