Why do my cookie sheets warp and make a loud pop in the oven?

Warping happens when thin metal heats unevenly the center expands faster than the edges, causing the sheet to flex. The "pop" is the metal snapping between buckled and flat positions. To prevent this: (1) Buy thicker, heavier sheets 18 gauge aluminum virtually never warps. (2) Avoid putting cold sheets in a very hot oven. (3) Don't set hot sheets on a cold surface. (4) Look for sheets with reinforced rims that resist flexing.