Final answer:
Running hot water over a twist off jar top helps loosen it by causing the metal lid to expand more than the glass jar and creating a gap between them.
Step-by-step explanation:
Running hot water over a stubborn twist off jar top can help loosen it due to the difference in thermal expansion between the metal lid and the glass jar. When hot water is poured over the lid, it causes the lid to expand more than the jar because metals have higher coefficients of expansion than glass. This expansion creates a slight gap between the lid and the jar, making it easier to twist off the lid.