Brown sugar doesn’t always behave. One day it’s perfectly soft and scoopable; the next, it’s a solid, unyielding block. Why does this happen, and how should you store brown sugar to avoid hard lumps?

Here's how to tell if that block of sugar is bad—or still plenty sweet enough to use. Unless you’re a very frequent baker, you’ve probably found a long-forgotten open bag of brown sugar lurking on one ...