It is best to boil eggs that are at room temperature as cracking occurs when chilled eggs are plunged into boiling water – the air in the air sac suddenly expands and causes the shell to crack.   Place the eggs in a pan of cold water, bring to the boil and simmer gently for 10 minutes.  

 

Cool under running water then leave to stand in cold water for 5 minutes.

Ever had an egg with an unpleasant grey rim around the yolk?  This happens when the eggs have been cooked for too long or not cooled quickly enough.  The iron in the yolk combines with sulphur in the white to produce hydrogen sulphide.  It’s harmless but spoils the appearance.