Why does my brown sugar keep turning into a brick?

Brown sugar hardens because it's basically white sugar coated in molasses, and the molasses holds the moisture. Once it dries out, it re-crystallizes into a brick. Your mom probably isn't doing anything magical, she just lives somewhere more humid or her kitchen has better conditions. Where I live it's dry enought that brown sugar turns to a rock in a couple months if I leave it in the bag.

The chip clip on the original bag isn't cutting it. What works is an airtight container, like a big jar or a plastic container with a real gasket lid. Even then, in dry climates it slowly loses moisture, so the classic trick is to store something damp in with it. A slice of bread is legit, not made up. The sugar pulls moisture from the bread as the bread dries out. Or you can use a terracotta sugar saver, the little disc you soak in water and then bury in the sugar. Same idea. Check on it every so often, the bread can get moldy if you forget about it. For the bags you already have, don't toss them. The fastest revival is the microwave: put the sugar in a bowl with a bit of water next to it or cover it loosely, microwave in short bursts and break it up with a fork between rounds. It softens fast but it re-hardens just as fast once it cools, so use it right away or move it to a proper container. The slower method that lasts is the bread trick, sealed up for a day or two. Worst case, if a bag is truly petrified, you can grate the chunks on a box grater or blitz them in a food processor and they work fine in a recipe.

Going forward, the container plus a sugar saver means you can buy one bag at a time instead of finding an archaeology site behind the flour canister.

I went through the same cycle of buying bag after bag before I figured out my kitchen is basically a desert. What finally fixed it for me was tossing a couple of marshmallows in the jar with the sugar. Sounds silly but they stay soft and the sugar does too. I've had the same jar going for close to a year now and it's still scoopable every time. The marshmallows go a bit stiff themselves but who cares, they're doing their job. Cheaper than the little terracotta discs too if you've already got a bag of them in the pantry. For your four rock bags, the microwave revival does work, just don't walk away from it or you'll end up with melted sugar syrup welded to the bowl.

I bake at a small cafe and we go through maybe 15 pounds of brown sugar a month, so it never sits long enough to harden on me at work. Home is a diffrent story though. My fix was just dating every bag with a sharpie when I open the cupboard, because half my problem was forgetting how old things were. Also for the petrified ones, don't bother chipping, just seal a chunk in a container overnight with a damp paper towel on a separate lid piece so it doesn't touch the sugar. Soft by morning.

I gave up on keeping it soft honestly. Now I just keep white sugar around and make brown sugar when I need it, a cup of white with a tablespoon or so of molasses stirred in with a fork. Takes two minutes and it's always fresh. That said, when I did have hardened bags, the food processor trick worked great for me too. Buzzed a whole brick into usable sugar in under a minute, no chipping involved. You already own the fix if you've got one.

My mother kept hers in a stoneware crock with a tight lid, and there was always a wedge of apple sitting on top. Replaced it every week or two and that sugar stayed scoopable till the crock was empty. Same principle as the bread trick but the apple never goes moldy as fast, you just eat the wedge or toss it. Learned that in her kitchen fifty years ago and it still works. I use a big glass jar now instead of the crock but the apple slice goes in every time. Haven't bought a sugar saver disc in my life.