Once a month my wife and I go grocery shopping. We used to spend three or four hours in the store, wandering the aisles, eyeballing amounts and grabbing extra just in case. Then back home it would turn out we already had bolognese sauce, and the vegetables were for a dish we never got around to making.

Now, before we go, we open an app where we've gradually added our usual dishes with the ingredients we actually use, not a pile of spices like in chef recipes. We pick what we roughly want to eat that month, mark what's already in the fridge, and get a ready shopping list.

Then something happened that we didn't expect at all. We were standing at the checkout, looking at the receipt and wondering if we'd forgotten something, because it was noticeably smaller than usual. We figured we got lucky with discounts or just bought less. But next time it happened again, and then again, and only then did it hit us: we were spending a third less on groceries, and the trip took an hour and a half instead of three or four.

The reason turned out to be ridiculously simple. When you know in advance what you're going to cook and can see what's already at home, all the "just in case" stuff disappears.