A tightly screwed lid on the jar is easier to unscrew if heated. Why?

The cover is made of metal. When heated, metal is plastic and expands, so the diameter of the lid increases and it becomes easier to open it. It also happens that new cans of food are screwed on at the cannery under pressure, so the lid is slightly bent inward. It is enough to pry the inner edge of the lid with a knife so that air gets into the can and the pressure is equalized, after which the lid can be easily removed.