Atight metal lid on a glass jar of jam may loosen if the lid is held under hot water for few minutes. this is because the water causes the

Heating causes things to expand.

During thermal expansion, every linear dimension increases by the same percentage with an increase in temperature, including holes. This assumes that the expanding material is uniform. Applied to a metal jar lid, this implies that the inside diameter of the lid will increase, tending to loosen the lid. Directing the stream of hot water at the center of the lid heats the metal more than the glass, so that you get a greater difference in the expansion of the top and the jar. The thermal conductivity of the metal top is much greater than that of the glass, and its specific heat is relatively low, so the temperature of the entire metal lid rises rapidly.