You have opened a bag and found soup in it. That is why you now pack sandwiches.
Leak anxiety is the reason most people stop bringing the food they actually want. Curry, stew, a grain bowl with dressing on it — all of it gets replaced by things that cannot betray you. The annoying part is that most leaks are preventable, and most of them are not the container’s fault.
Why things leak
Heat. This is the big one and almost nobody knows it. Hot food sealed in a container cools, the air inside contracts, and the pressure difference works against the seal. A container that holds perfectly at room temperature can weep when you fill it straight off the hob.
No headspace. Fill to the rim and there is nowhere for anything to go. The seal takes the whole load.
A dirty gasket. A seal with a fleck of rice under it is not a seal. This is the single most common cause of a container that “suddenly started leaking.”
A worn gasket. Silicone is consumable. After enough years it stiffens and stops springing back.
The fixes, in order of how much they matter
Let it cool first. Warm, not hot, before the lid goes on. If you have to seal it hot, leave more room and keep it upright. This one change fixes most leaks.
Leave headspace. Around four-fifths full for anything liquid. In an 800ml container that’s roughly 650ml of soup. Same rule as the freezer, different reason.
Clean under the seal. Take the ring out, wash it, dry it, put it back. Once a week if you carry wet food daily. It takes ten seconds and it prevents most of what people blame the container for.
Keep dressing separate. A 400ml round for the dressing, a bigger one for the leaves, combine at the desk. Better salad, and if something does go wrong it goes wrong inside one small container instead of across a bag.
Pack it upright. A good seal should hold on its side. There is no reason to find out.
Which shape
Round containers are better for anything wet — there are no corners for the seal to negotiate, and the pressure sits evenly around the ring. Rectangles stack better and are the right call for a packed lunch that isn’t swimming.
That’s the trade. Wet food, round. Everything else, rectangular.
The honest part
We say leak-resistant, not leak-proof, and that’s deliberate. A silicone seal under compression is very good and it is not a promise. Anyone selling you “leakproof” on a container with a removable gasket is describing a new container, not a container in year three.
What we can tell you is which part fails and what to do about it. It’s the seal, it’s always the seal, and a new one costs €4.95 rather than the price of starting again. Two materials, both named — 304 stainless steel and that ring — and we name it precisely because it’s the part with a shorter life than the box around it.
Start with one thing this week. A dressing, a yoghurt, a small soup. Five uneventful days and the anxiety goes, and you get your lunch options back.
Food should be stored without doubt.