I used to throw away half a bag of brown sugar every time I baked. Not because I do not bake often — because every time I opened the bag, it was a brick. A literal brick. I chiseled at it with a butter knife once and nearly stabbed myself.

Then my neighbor — a retired pastry chef — walked over while I was aggressively hammering a bag of brown sugar on the counter and said three words that changed everything: marshmallows. Just marshmallows.

Why brown sugar turns into a rock

Brown sugar is just white sugar with molasses mixed back in. When the molasses loses moisture, the whole thing hardens. Commercial brown sugar dries out fast because the packaging is not airtight once you open it. The sugar crystals lock together and suddenly you need a chisel instead of a measuring cup.

The fix is not complicated. You just need to put moisture back into the container — and keep it there.

The marshmallow trick

Toss two or three regular marshmallows into your brown sugar container and seal it. That is it. No fridge. No plastic. The marshmallows slowly release moisture into the sealed environment and the sugar stays soft. They will shrink and harden over time — that means they are doing their job. When they get small and crunchy, swap in fresh ones.

I have had the same bag of brown sugar for four months now. I reach in and it pours like sand. My butter knife has been peacefully unemployed.

Other things that also work

• A slice of bread: Works the same way as marshmallows. Toss a slice in, seal, wait overnight. The bread dries out and the sugar softens up. Replace the bread slice every few days so it does not mold.
• A damp paper towel: Faster but riskier. Lay a damp paper towel over the top of the sugar, not touching it directly. Seal the container for a few hours. Do not leave it too long or you get wet sugar instead of soft sugar.
• An apple slice: Same concept. A fresh apple slice releases moisture. Just check it daily so it does not start fermenting in your sugar.

What does not work (I tried)

Microwaving it. I put a chunk of brown sugar in the microwave with a damp paper towel and got a half-melted, half-rock-hard disaster. The microwave heats unevenly and you cannot control where the moisture goes. Also: the terracotta disc things they sell at kitchen stores. I bought one for eight dollars. It worked for about a week and then stopped. Marshmallows cost two dollars and last months.

Prevention is even easier

Once your sugar is soft, keep it in an airtight container in a cool cupboard. Not the fridge — the fridge is dry and cold and makes it harden faster. Just a sealed container at room temperature with a marshmallow or two inside. That is the whole system.

I have not thrown out a bag of brown sugar in over a year. That retired pastry chef saved me more money than I want to think about. I bake more often now because I am not dreading the ten-minute chisel session before I even preheat the oven.