Put a chewy cookie and a crisp one in the same jar on Monday. By Thursday the chewy one is hard and the crisp one has gone limp. Both were stored correctly. Neither is stale in the way people mean when they say stale. They traded.
Two opposite failures, on the same shelf
A soft cookie and a crisp cookie fail in opposite directions, and understanding which one you are holding tells you everything about how to keep it.
A soft cookie goes hard. Two things happen at once. It loses water to the air around it, because it holds more moisture than the room does. At the same time the starch in the flour recrystallises as it sits, the same process that stales bread, and that pushes out more of the water still inside. The cookie ends up drier and firmer than it started even though nothing was done to it.
A crisp cookie goes soft. The opposite. A crisp cookie or a cracker was baked to a low moisture content, and it is full of sugar and salt, both of which are hygroscopic. They pull water vapour out of the air. Left uncovered in a normal kitchen it takes days at most for the snap to go.
So the same jar, in the same room, is doing two different jobs. And if both types are in it, the jar is not a neutral container. It is a channel.
Why a mixed jar ruins both
Inside a closed jar the air reaches a single humidity. It has to, because it is one pocket of air. Whatever is wettest in there gives up moisture until the air is satisfied, and whatever is driest takes it in.
So the chewy cookie becomes the donor. It hands its moisture to the air, the air hands it to the crisp cookie, and the crisp cookie takes it because sugar and salt will take it from anywhere. The chewy one goes hard faster than it would have alone, and the crisp one goes soft faster than it would have alone. Both are worse for having been stored together, and neither has done anything wrong.
This is why the single most useful storage rule for baking has nothing to do with the container. One type of cookie per jar. If you have made two kinds, use two jars, or a jar and a tin, or a jar and a bag. It costs nothing and it is worth more than any lid.
What airtight is actually doing
An airtight seal does not stop either process. It slows the exchange with the room, which is a smaller claim and a true one.
For a crisp cookie that is most of the battle, because the room is the source of the moisture that softens it. Seal it away from the kitchen and it stays crisp for a long time. This is the case where a good gasket earns its keep, and it is why our cookie jar has one rather than a lid that merely sits.
For a soft cookie it helps less than people hope. Sealing stops the water leaving, but it does nothing about the starch recrystallising, and that is happening inside the cookie regardless of what is around it. A sealed soft cookie stays moist and still firms up. It just firms up more slowly and without drying out, which is a better outcome but not the one people expect.
The slice of bread, and why it works
The old trick is to put a slice of bread in with soft cookies. It is not folklore. Bread has a much higher moisture content than a baked cookie, so in a sealed jar it becomes the donor and the cookies stop being it. The air reaches its humidity from the bread instead, and the cookies hold what they have.
Two things about it that usually go unsaid. It only works in a genuinely closed jar, because in an open one the room wins and the bread just goes stale on its own. And it works exactly as well in reverse, which is the part worth knowing: never put a slice of bread in with crisp cookies, because you will have soft ones by morning.
Change the slice every couple of days. A slice that has given up its moisture is doing nothing, and one left a week is doing something else.
Reviving either one
A crisp cookie that has gone soft is easy. Five to eight minutes in an oven at around 300 degrees Fahrenheit drives the absorbed water back off, then let it cool completely on a rack before it goes anywhere, because it crisps as it cools and not in the oven.
A soft cookie that has gone hard is harder, and it depends which failure you have. If it dried out, the bread trick in a sealed jar overnight will bring some of it back. If the starch has recrystallised, only heat reverses that, which means a short warm oven and eating it the same day. That is the same mechanism as reviving a stale loaf, described in how to keep sourdough fresh.
The honest limit
An airtight jar cannot fix a mixed jar. That is the failure we see described as a storage problem most often, and it is not one. It is two products with opposite needs sharing one pocket of air, and the better the seal, the more completely they equalise. A perfect gasket makes a mixed jar worse, not better.
It also will not hold a soft cookie at day-one texture. Nothing will, because the starch does not know it is in a jar. What it will do is keep a crisp cookie snapping for weeks instead of days, keep a soft one from drying out on top of firming up, and keep both away from the smell of whatever was cooked on Tuesday.
The lid is solid bamboo and wants a little looking after, which is covered in how to look after a bamboo lid. The glass is borosilicate, and why that matters is in care and materials.