A few minutes after sliding a tray of cookie dough into the oven, you hear a thunderous clang from inside. Even if you’ve lived through the experience dozens of times, it can still be pretty startling.
This happens because metal expands when it’s suddenly exposed to the high heat of the oven. But it’s not a totally uniform process; some parts of the pan might expand farther or faster than others. If the pressure is too much for the sheet to bear, it can get bent out of shape—which causes the loud bang that makes you jump out of your skin.
Thicker pans have an easier time absorbing the stress of heat expansion, so buying a “commercial” or “double-thick” aluminum baking sheet can help you avoid the issue. The noise itself isn’t the only problem: A warped pan is hard to handle and also bad for baking or roasting food evenly. As the Basically blog explains, a rolled-edge rim is just as important as thickness in preventing misshapen sheets. It supports the edges of the pan, forces the bottom to remain flat, and helps the whole pan heat up at the same rate. Even a nice, thick, rolled-edged baking sheet could make a little noise in the oven, but it’ll more easily return to its original shape once the temperature evens out.
Without any reinforcement, thin, rimless sheets are more likely to get warped and then stay that way—a little like how it would be much harder to bend a plastic box out of shape than it would be to twist its flimsy lid until it’s no longer a flat surface.