Why Is the Last Stair Always the One That Gets You?

You have climbed these stairs several thousand times. Tonight your foot arrives at the top, finds a step that is not there, and your whole body performs that small ungraceful jolt which has no dignified version. It is always the last stair. Nobody trips on the third one.

The reason is that your legs are not counting. After two or three steps the job is handed to a motor programme — a small rhythm that runs on its own, freeing you up to think about the fridge. A staircase is a metronome you can walk up, and the jolt is what happens when the music stops one beat before your body expected it to.

There is neat evidence for this. Step onto a broken escalator, one you can plainly see is switched off and standing perfectly still, and you will lurch anyway. That lurch has been measured in laboratories: the body applies a correction for a movement that is not going to happen, even while you are looking straight at the motionless steps and telling yourself they are motionless. Knowing makes no difference whatsoever. The forecast was filed long before you arrived.

Which makes the last stair less a failure than a compliment. Your body built a model of the world out of the first few steps, ran it faithfully in the dark, and was right about every single one until the staircase ran out of evidence. The stumble is the sound of a good prediction meeting the end of the data. The correct response is to apologise to the stairs, quietly, and carry on to bed.