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.