Baking sheets are exposed to high temperatures in the broiler – often 500°F to 550°F. Heat causes metal to expand, but the sheet’s edges don’t always expand at the same rate due to temperature variations in the oven. Because it’s thinner and sitting on hot metal bars, the sheet’s bottom will expand more quickly than the raised edges and lip. This imbalance is likely to cause thinner sheets to twist, buckle, or warp. Sheets that repeatedly twist or buckle may crack over time. To prevent this, choose thicker sheets or ones with rolled, wire-reinforced rims. They may still bend slightly due to temperature differences, but they will return to normal once the temperature stabilizes.