Why Can Warm Water Make a Metal Jar Lid Easier to Open?

When a metal lid will not come off a glass jar, directing warm water mainly over the lid can help. The immediate reason is thermal expansion: most solids become slightly larger as they warm, and common metals generally expand more than ordinary glass. A lid is also a metal ring with an opening. When heated, its inner diameter grows along with its outer edge, so its fit around the glass thread may loosen slightly.

The dimensional change is tiny, but it can still reduce the torque needed to start the lid moving because the seal may already be close to slipping. Metal also conducts heat quickly, so during a short rinse the lid usually warms faster than the thicker glass body. A sealing liner may soften as well. If the entire jar is heated evenly, both lid and glass expand, so the effect is usually less pronounced than heating the lid selectively.

Warm water does not simply “remove friction”, and the method will not solve every problem. Dried syrup, corrosion or a distorted lid can create different kinds of resistance. Warm tap water is enough; dry the lid before twisting it. Do not pour boiling water suddenly over very cold, damaged or cracked glass, because rapid uneven heating can cause thermal shock.

Source: https://openstax.org/books/college-physics-2e/pages/13-2-thermal-expansion-of-solids-and-liquids