Packing at the till
Everyone knows the rule, and nobody was ever taught it. Heavy things go at the bottom. Tins and bottles, then boxes, then the soft things, and the bread and eggs go on top, where nothing can land on them. It is the one piece of engineering most people do every week, and most weeks they still get it wrong. It isn’t for lack of knowing. It is the order the things arrive in.
At a staffed till the shopping comes down the chute one item at a time, in whatever order the cashier picks it up. That is roughly the order you put it on the belt, and you put it on the belt in roughly the order it came out of the trolley. So the first thing to reach the bag is often the last thing you picked up, which is often the bread, because you remembered it at the end. The rule wants the bag built from the bottom. The belt hands it over from the top.
A packer with time would just let things pile up in the chute and choose from them. That is what the chute is for. But the chute is short, and it isn’t yours for long. The cashier is fast, and in many chains how fast is something that gets measured. The next customer’s shopping is already on the belt behind the divider. There is a moment, somewhere around the milk, when the pile is outrunning your hands. From then on you are not packing. You are clearing. Things go into whichever bag is open, in the order they come, and the eggs end up under the potatoes.
Then the card machine asks for you while both hands are full. Paying is the one step that can’t be done in parallel with packing, so it goes in the middle of it. The cashier waits, the queue waits, and the half-packed bag sits there being wrong.
Some discount chains have taken the race away entirely. The cashier sits, scans at a speed nobody could pack at, and sends everything straight back into your trolley. Packing happens afterwards at a shelf by the window, out of the queue, at your own pace. It looks brusque the first time. It is the only version of the till where the rule can actually be followed, because it is the only one where you get to see all the shopping before the first thing goes in the bag.
The self-checkout does weigh the bag. Every item scanned is expected to arrive on the scale beside it, and a weight it wasn’t expecting stops the whole machine. But the scale is checking the bag against the receipt. It has no view on whether the bread is underneath the tins.
People who pack well have usually worked out where the job really starts. They unload the trolley heavy-first: bottles and tins at the front of the belt, the bread last, with a gap before the divider. The cashier does nothing different and the chute still runs too fast. But now the order it hands over is the order the bag needs, and clearing and packing are the same thing. Whatever ends up at the bottom of the bag was decided at the belt, by someone who wasn’t thinking about the bag yet.