Why do you use hot water to remove a tight metal cap easily?

Often, lids are made of metals which are great conductors of electromagnetic waves (including heat). When warm water is applied to a "difficult" lid, excitement of the atoms in the metal occurs (very slight) which causes a slight expansion. This expansion helps the lid to be moved from it's grip on the jar or bottle. This is also found in plastics. Although not through expansions. The heat from the warm water literally softens the plastic.