You peel a perfectly normal hard-boiled egg expecting a bright yellow yolk, only to find a strange greenish-gray ring wrapped around it. This unappetizing discoloration causes thousands of people each year to worry that the egg has spoiled, gone bad, or become unsafe to eat.

In reality, the green ring is completely harmless and is caused by a simple chemical reaction triggered when eggs are overcooked or exposed to excessively high heat. When this happens, the naturally occurring sulfur in the egg white reacts with the iron in the yolk, creating a compound called iron sulfide.

This discoloration has absolutely nothing to do with the freshness of the egg, though overcooking will often release a strong sulfur smell alongside it. Fortunately, preventing the ring is easy: start the eggs in cold water, bring them to a gentle boil, turn off the heat to let them finish cooking via residual heat, and time them carefully for 9 to 12 minutes.

The final, crucial step is immediate cooling by transferring the cooked eggs into an ice water bath. This rapidly stops the internal cooking process—preventing the iron-sulfur reaction from taking place—while simultaneously shrinking the egg slightly inside the shell to make peeling much easier.