Expansion rings are the rings stamped onto the end of a can that expand slightly during heat processing — the concentric ridges pressed into can lids. During retorting, the contents heat up, pressure builds inside the sealed can, and the rings flex outward to absorb the stress, then relax as the can cools — a built-in expansion joint for the thermal cycle. Without them, the pressure would buckle the flat end or break the seal. The rings also serve inspection: a can whose ends bulge permanently has internal gas pressure from microbial spoilage — the swollen tin that must never be opened. It is a small, elegant piece of packaging engineering: stamped geometry doing the work of a pressure valve, protecting the seal through the violence of sterilisation.